The appearance of the myelin sheath can be thought of as similar to the pastry wrapped around a hot dog for “pigs in a blanket” or similar food. The glial cell is wrapped around the axon several times with little to no cytoplasm between the glial cell layers. For oligodendrocytes, the rest of the cell is separate from the myelin sheath as a cell process extends back toward the cell body. A few other processes provide the same insulation for other axon segments in the area. For Schwann cells, the outermost layer of the cell membrane contains cytoplasm and the nucleus of the cell as a bulge on one side of the myelin sheath. During development, the glial cell is loosely or incompletely wrapped around the axon. The edges of this loose enclosure extend toward each other, and one end tucks under the other. The inner edge wraps around the axon, creating several layers, and the other edge closes around the outside so that the axon is completely enclosed.


The nervous system is involved in receiving information about the environment around us (sensation) and generating responses to that information (motor responses). The nervous system can be divided into regions that are responsible for sensation (sensory functions) and the response (motor functions), but there is a third function that needs to be included. Sensory input needs to be integrated with other sensations, as well as with memories, emotional state, or learning (cognition). Some regions of the nervous system are termed integration or association areas. The process of integration combines sensory perceptions and higher cognitive functions such as memories, learning, and emotion to produce a response.


The first major function of the nervous system is sensation—receiving information about the environment to gain input about what is happening outside the body (or, sometimes, within the body). The sensory functions of the nervous system register the presence of a change from homeostasis or a particular event in the environment, known as a stimulus. The senses we think of most are the “big five”: taste, smell, touch, sight, and hearing. The stimuli for taste and smell are both chemical substances (molecules, compounds, ions, etc.), touch is physical or mechanical stimuli that interact with the skin, sight is light stimuli, and hearing is the perception of sound, which is a physical stimulus similar to some aspects of touch. There are more senses than just those, but that list represents the major senses. Those five are all senses that receive stimuli from the outside world, and of which there is conscious perception. Additional sensory stimuli might be from the internal environment (inside the body), such as the stretch of an organ wall or the concentration of certain ions in the blood.


The nervous system produces a response on the basis of the stimuli perceived by sensory structures. An obvious response would be the movement of muscles, such as withdrawing a hand from a hot stove, but there are broader uses of the term. The nervous system can cause the contraction of all three types of muscle tissue. For example, skeletal muscle contracts to move the skeleton, cardiac muscle is influenced as heart rate increases during exercise, and smooth muscle contracts as the digestive system moves food along the digestive tract. Responses also include the neural control of glands in the body as well, such as the production and secretion of sweat by the eccrine and merocrine sweat glands found in the skin to lower body temperature.


Responses can be divided into those that are voluntary or conscious (contraction of skeletal muscle) and those that are involuntary (contraction of smooth muscles, regulation of cardiac muscle, activation of glands). Voluntary responses are governed by the somatic nervous system and involuntary responses are governed by the autonomic nervous system, which are discussed in the next section.


Stimuli that are received by sensory structures are communicated to the nervous system where that information is processed. This is called integration. Stimuli are compared with, or integrated with, other stimuli, memories of previous stimuli, or the state of a person at a particular time. This leads to the specific response that will be generated. Seeing a baseball pitched to a batter will not automatically cause the batter to swing. The trajectory of the ball and its speed will need to be considered. Maybe the count is three balls and one strike, and the batter wants to let this pitch go by in the hope of getting a walk to first base. Or maybe the batter’s team is so far ahead, it would be fun to just swing away.


The autonomic nervous system (ANS) is responsible for involuntary control of the body, usually for the sake of homeostasis (regulation of the internal environment). Sensory input for autonomic functions can be from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue. The role of the autonomic system is to regulate the organ systems of the body, which usually means to control homeostasis. Sweat glands, for example, are controlled by the autonomic system. When you are hot, sweating helps cool your body down. That is a homeostatic mechanism. When you are nervous, you might start sweating also. That is not homeostatic, it is the physiological response to an emotional state.


There is another division of the nervous system that describes functional responses. The enteric nervous system (ENS) is responsible for controlling the smooth muscle and glandular tissue in your digestive system. It is a large part of the PNS, and is not dependent on the CNS. It is sometimes valid, however, to consider the enteric system to be a part of the autonomic system because the neural structures that make up the enteric system are a component of the autonomic output that regulates digestion. There are some differences between the two, but for our purposes here there will be a good bit of overlap. See Figure 8.13 for examples of where these divisions of the nervous system can be found.


The cerebrum is the seat of many of the higher mental functions, such as memory and learning, language, and conscious perception, which are the subjects of subtests of the mental status exam. The cerebral cortex is the thin layer of gray matter on the outside of the cerebrum. It is approximately a millimeter thick in most regions and highly folded to fit within the limited space of the cranial vault. These higher functions are distributed across various regions of the cortex, and specific locations can be said to be responsible for particular functions. There is a limited set of regions, for example, that are involved in language function, and they can be subdivided on the basis of the particular part of language function that each governs.


Assessment of cerebral functions is directed at cognitive abilities. The abilities assessed through the mental status exam can be separated into four groups: orientation and memory, language and speech, sensorium, and judgment and abstract reasoning.


Orientation is the patient’s awareness of his or her immediate circumstances. It is awareness of time, not in terms of the clock but of the date and what is occurring around the patient. It is awareness of place, such that a patient should know where he or she is and why. It is also awareness of who the patient is—recognizing personal identity and being able to relate that to the examiner. The initial tests of orientation are based on the questions, “Do you know what the date is?” or “Do you know where you are?” or “What is your name?” Further understanding of a patient’s awareness of orientation can come from questions that address remote memory, such as “Who is the President of the United States?”, or asking what happened on a specific date.


Memory is largely a function of the temporal lobe, along with structures beneath the cerebral cortex such as the hippocampus and the amygdala. The storage of memory requires these structures of the medial temporal lobe. A famous case of a man who had both medial temporal lobes removed to treat intractable epilepsy provided insight into the relationship between the structures of the brain and the function of memory.


The prefrontal cortex can also be tested for the ability to organize information. In one subtest of the mental status exam called set generation, the patient is asked to generate a list of words that all start with the same letter, but not to include proper nouns or names. The expectation is that a person can generate such a list of at least 10 words within 1 minute. Many people can likely do this much more quickly, but the standard separates the accepted normal from those with compromised prefrontal cortices.


Language is, arguably, a very human aspect of neurological function. There are certainly strides being made in understanding communication in other species, but much of what makes the human experience seemingly unique is its basis in language. Any understanding of our species is necessarily reflective, as suggested by the question “What am I?” And the fundamental answer to this question is suggested by the famous quote by René Descartes, “Cogito Ergo Sum” (translated from Latin as “I think, therefore I am”). Formulating an understanding of yourself is largely describing who you are to yourself. It is a confusing topic to delve into, but language is certainly at the core of what it means to be self-aware.


The neurological exam has two specific subtests that address language. One measures the ability of the patient to understand language by asking them to follow a set of instructions to perform an action, such as “touch your right finger to your left elbow and then to your right knee.” Another subtest assesses the fluency and coherency of language by having the patient generate descriptions of objects or scenes depicted in drawings, and by reciting sentences or explaining a written passage.


An important example of multimodal integrative areas is associated with language function (see Figure 8.14). Adjacent to the auditory association cortex, at the end of the lateral sulcus just anterior to the visual cortex, is Wernicke’s area. In the lateral aspect of the frontal lobe, just anterior to the region of the motor cortex associated with the head and neck is Broca’s area. Both regions were originally described on the basis of losses of speech and language, which is called aphasia. The aphasia associated with Broca’s area is known as expressive aphasia, which means that speech production is compromised. This type of aphasia is often described as non-fluency because the ability to say some words leads to broken or halting speech. Grammar can also appear to be lost. The aphasia associated with Wernicke’s area is known as receptive aphasia, which is not a loss of speech production but a loss of understanding of content. Patients, after recovering from acute forms of this aphasia, report not being able to understand what is said to them or what they are saying themselves, but they often cannot keep from talking.


The two regions are connected by white matter tracts that run between the posterior temporal lobe and the lateral aspect of the frontal lobe. Conduction aphasia associated with damage to this connection refers to the problem of connecting the understanding of language to the production of speech. This is a very rare condition but is likely to present as an inability to faithfully repeat spoken language.


Those parts of the brain involved in the reception and interpretation of sensory stimuli are referred to collectively as the sensorium. The cerebral cortex has several regions that are necessary for sensory perception. Several of the subtests can reveal activity associated with these sensory modalities, such as being able to hear a question or see a picture. Two subtests assess specific functions of these cortical areas.


The first is praxis, a practical exercise in which the patient performs a task completely on the basis of verbal description without any demonstration from the examiner. The second subtest for sensory perception is gnosis, which involves two tasks. The first task, known as stereognosis, involves the naming of objects strictly on the basis of the somatosensory information that comes from manipulating them. The patient keeps their eyes closed and is given a common object, such as a coin, that they have to identify. The patient should be able to indicate the particular type of coin, such as a dime versus a penny, or a nickel versus a quarter, on the basis of the sensory cues involved. For example, the size, thickness, or weight of the coin may be an indication, or to differentiate the pairs of coins suggested here, the smooth or corrugated edge of the coin will correspond to the particular denomination. The second task, graphesthesia, is to recognize numbers or letters written on the palm with a dull pointer, such as a pen cap.


The prefrontal cortex is responsible for the functions responsible for planning and making decisions. In the mental status exam, the subtest that assesses judgment and reasoning is directed at three aspects of frontal lobe function. First, the examiner asks questions about problem-solving, such as “If you see a house on fire, what would you do?” The patient is also asked to interpret common proverbs, such as “Don’t look a gift horse in the mouth.” Additionally, pairs of words are compared for similarities, such as apple and orange, or lamp and cabinet.


The reason for this is that the language functions of the cerebral cortex are localized to the left hemisphere in 95% of the population. Additionally, the left hemisphere is connected to the right side of the body through the corticospinal tract and the ascending tracts of the spinal cord. Motor commands from the precentral gyrus control the opposite side of the body, whereas sensory information processed by the postcentral gyrus is received from the opposite side of the body. For a verbal command to initiate movement of the right arm and hand, the left side of the brain needs to be connected by the corpus callosum. Language is processed in the left side of the brain and directly influences the left brain and right arm motor functions but is sent to influence the right brain and left arm motor functions through the corpus callosum. Likewise, the left-handed sensory perception of what is in the left pocket travels across the corpus callosum from the right brain, so no verbal report on those contents would be possible if the hand happened to be in the pocket.


Neurodegenerative Diseases – Alzheimer’s disease, Parkinson’s disease, Amyotrophic Lateral Sclerosis (ALS), Multiple sclerosis (MS)


Several diseases can result from the demyelination of axons. The causes of these diseases are not the same; some have genetic causes, some are caused by pathogens, and others are the result of autoimmune disorders. Though the causes are varied, the results are largely similar. The myelin insulation of axons is compromised, making electrical signaling slower.


Multiple sclerosis (MS) is one such disease. It is an example of an autoimmune disease. The antibodies produced by lymphocytes (a type of white blood cell) mark myelin as something that should not be in the body. This causes inflammation and the destruction of the myelin in the central nervous system. As the insulation around the axons is destroyed by the disease, scarring becomes obvious.


Guillain-Barre (pronounced gee-YAN bah-RAY) syndrome is an example of a demyelinating disease of the peripheral nervous system. It is also the result of an autoimmune reaction, but the inflammation is in peripheral nerves. Sensory symptoms or motor deficits are common, and autonomic failures can lead to changes in the heart rhythm or a drop in blood pressure, especially when standing, which causes dizziness.


Infection, trauma, and congenital disorders can all lead to significant signs, as identified through the neurological exam. It is important to differentiate between an acute event, such as stroke, and a chronic or global condition such as blunt force trauma. Responses seen in the neurological exam can help. A loss of language function observed in all its aspects is more likely a global event as opposed to a discrete loss of one function, such as not being able to say certain types of words. A concern, however, is that a specific function—such as controlling the muscles of speech—may mask other language functions. The various subtests within the mental status exam can address these finer points and help clarify the underlying cause of the neurological loss.


Damage to the nervous system can be limited to individual structures or can be distributed across broad areas of the brain and spinal cord. Localized, limited injury to the nervous system is most often the result of circulatory problems. The loss of blood flow to part of the brain is known as a stroke, or a cerebrovascular accident (CVA). There are two main types of stroke, depending on how the blood supply is compromised: ischemic and hemorrhagic. An ischemic stroke is the loss of blood flow to an area because vessels are blocked or narrowed. This is often caused by an embolus, which may be a blood clot or fat deposit. Ischemia may also be the result of thickening of the blood vessel wall, or a drop in blood volume in the brain known as hypovolemia. A hemorrhagic stroke is bleeding into the brain because of a damaged blood vessel. Accumulated blood fills a region of the cranial vault and presses against the tissue in the brain (see Figure 8.15).


Cerebral palsy (CP) is caused by an interruption to the normal development of a person’s brain, leading to weakness with muscles. Depending on the area of the brain that is affected, signs and symptoms will vary in the type and severity between individuals. Balance and coordination are often challenging due to the inability to control muscles (Centers for Disease Control and Prevention, n.d.-a). To learn more about cerebral palsy, please visit the Centers for Disease Control and Prevention.


According to the Centers for Disease Control and Prevention, about 166 people in the United States died each day from a traumatic brain injury in 2019. Brain injuries range from mild to severe and include concussions. TBI can be caused by falls, automobile accidents, assaults, and firearm-related suicide (Centers for Disease Control and Prevention, n.d.-b). To learn more about TBI, please visit the Centers for Disease Control and Prevention.


Neurologists are medical doctors who complete specialized training in the prevention, diagnosis, and treatment of disorders and conditions related to the brain and nervous system (Bureau of Labor Statistics, 2021). For more details visit the American Academy of Neurology.


Electromyography (EMG) is a procedure that assesses the electrical signals muscles send while at rest and when they are used. During the test, a needle electrode is placed into the muscle, and a machine records the muscle activity. EMG can be used to diagnose myasthenia gravis, muscular dystrophy, and other conditions affecting the muscles (MedlinePlus, 2021a). To learn more, please visit the Medline Plus web page on electromyography.


With electrodes applied to your scalp, an electroencephalogram (EEG) measures electrical activity in the brain. It’s used to help diagnose conditions of the brain, including seizures, altered mental status, and hemorrhage. (Rayi & Murr, 2021). For more information, please visit the Mayo Clinic’s web page on electroencephalograms.


Centers for Disease Control and Prevention. (n.d.-a). What is cerebral palsy? https://www.cdc.gov/ncbddd/cp/facts.html


Centers for Disease Control and Prevention. (n.d.-b). Get the facts about TBI. https://www.cdc.gov/traumaticbraininjury/get_the_facts.html


ClinicalInfo. (n.d.). Spinal tap. National Institute of Health Office of AIDS Research, U.S. Department of Health and Human Services. https://clinicalinfo.hiv.gov/en/glossary/spinal-tap


CrashCourse. (2015, February 23). The nervous system, part 1: Crash course A&P #8 [Video]. YouTube. https://www.youtube.com/watch?v=qPix_X-9t7E


MedlinePlus. (2021a). Electromyography (EMG) and nerve conduction studies. U.S. Library of Medicine, U.S. Department of Health and Human Services. https://medlineplus.gov/lab-tests/electromyography-emg-and-nerve-conduction-studies


Rayi, A., & Murr, N. (2021). Electroencephalogram. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK563295/


When most people think of muscles, they think of the muscles that are visible just under the skin, particularly of the limbs. These are skeletal muscles, so-named because most of them move the skeleton, but there are two additional types of muscles: the smooth muscle and the cardiac muscle. The body has over 600 muscles which contribute significantly to the body’s weight.


Media 7.1 Muscles, Part 2 – Organismal Level: Crash Course A&P #22 [Online video]. Copyright 2015 by CrashCourse.


Muscle is one of the four primary tissue types of the body, and it is made up of specialized cells called fibers. The body contains three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle (see Figure 7.1). All three muscle tissues have some properties in common; they all exhibit a quality called excitability as their plasma membranes can change their electrical states (from polarized to depolarized) and send an electrical wave called an action potential along the entire length of the membrane. Fascia is fibrous connective tissue that encloses muscles.


There are many nomenclatures for naming muscles. Some of these include:


Cerebral palsy (CP) is caused by an interruption to the normal development of a person’s brain leading to weakness with muscles. Depending on the area of the brain that is affected, signs and symptoms will vary in the type and severity between individuals. Balance and coordination are often challenging due to the inability to control muscles (Centers for Disease Control and Prevention, n.d.). To learn more, please visit the Centers for Disease Control and Prevention’s web page on cerebral palsy.


Carpal tunnel syndrome may present with pain, numbness, or weakness to the hand(s) caused by pressure on the median nerve. Some causes for this pressure are repetitive movements, trauma or injury to the wrist, or fluid retention related to pregnancy or menopause (National Institute of Neurological Disorders and Stroke, 2020). To learn more, visit the National Institute of Neurological Disorders and Stroke web page on carpal tunnel.


To learn more about paralysis, please visit the Cleveland Clinic’s web page on paralysis.


A sprain is an injury to a joint whereby a ligament is stretched or torn. Joints can be sprained as a result of falling, twisting, or being hit. Sprains most often occur in the ankle, although other joints can be affected. Signs and symptoms of a sprain include pain, swelling, bruising, and an inability to use the joint (National Institute of Arthritis and Musculoskeletal and Skin Diseases, n.d.).


A strain is an injury to a muscle or a tendon caused by stretching or tearing. Tendons or muscles can be strained as a result of an injury, lifting heavy objects incorrectly, or overstress, and they can develop suddenly or over time. Signs and symptoms of a strain include pain, muscle spasms, swelling, cramping, and difficulty moving the muscle (National Institute of Arthritis and Musculoskeletal and Skin Diseases, n.d.).


Orthopedic surgeons are medical doctors who have specialized training in the prevention, diagnosis, treatment, and surgery of disorders and diseases related to the musculoskeletal systems (Bureau of Labor Statistics, 2021). For more details, please visit the American College of Surgeons’ web page on orthopedic surgery.


Massage therapists manipulate muscles and other soft tissues through touch to relieve pain, aid the injury-healing process, and reduce stress. Massage therapists generally have a postsecondary degree, although requirements vary by state (Bureau of Labor Statistics, 2021b). To learn more about massage therapy, visit the American Massage Therapy Association’s web page.


Magnetic Resonance Imaging (MRI) is a test that uses radio waves and a magnetic field to view internal organs and structures. MRI tests are used to diagnose a variety of conditions, such as torn ligaments or tumors. They are also used to view the brain and spinal cord (Medline Plus, 2021b).


Range of Motion Testing is a diagnostic procedure used to determine the amount of movement around a specific joint.


Involuntary and found only in the heart. Highly coordinated contractions pump blood into the vessels of the circulatory system.


Recording of muscle electrical activity in response to a nerve’s stimulation of the muscle.


A general term for the group of inherited myopathies that are characterized by wasting and weakness of the skeletal muscle.


A disease in which antibodies made by a person’s immune system prevent certain nerve-muscle interactions, causing weakness in the arms and legs, vision problems, and drooping eyelids or head.


Paralysis that affects both legs and lower part of the body.


Partial paralysis wherein there is still some control of the muscles.


An inflammatory disease of the muscles closest to the center of the body.


The muscles responsible for voluntary muscle movement; also called striated muscle.


The muscles responsible for involuntary muscle movement; also called visceral muscle.


Bureau of Labor Statistics. (2021a). Physicians and surgeons. In Occupational outlook handbook. U.S. Department of Labor. https://www.bls.gov/ooh/healthcare/physicians-and-surgeons.htm


Bureau of Labor Statistics. (2021b). Massage therapists. In Occupational outlook handbook. U.S. Department of Labor. https://www.bls.gov/ooh/healthcare/massage-therapists.htm


Centers for Disease Control and Prevention. (n.d.). What is cerebral palsy? https://www.cdc.gov/ncbddd/cp/facts.html


CrashCourse. (2015, July 15). Muscles, part 2 – organismal level: Crash course A&P #22 [Video]. YouTube. https://youtu.be/I80Xx7pA9hQ


National Library of Medicine. (2021). Medical Subject Headings database. National Institutes of Health, U.S. Department of Health and Human Services. https://www.nlm.nih.gov/mesh/meshhome.html


MedlinePlus. (2021a). Electromyography (EMG) and nerve conduction studies. U.S. National Library of Medicine. https://medlineplus.gov/lab-tests/electromyography-emg-and-nerve-conduction-studies


MedlinePlus. (2021b). MRI scans. U.S. National Library of Medicine, U.S. Department of Health and Human Services. https://medlineplus.gov/mriscans.html


National Human Genome Research Institute (2013). About Duchenne Muscular Dystrophy. National Institutes of Health, U.S. Department of Health and Human Services. https://www.genome.gov/Genetic-Disorders/Duchenne-Muscular-Dystrophy


National Institute of Arthritis and Musculoskeletal and Skin Diseases. (n.d.). Sprain vs. strain. National Institutes of Health, U.S. Department of Health and Human Services. https://www.niams.nih.gov/health-topics/sprains-and-strains#tab-symptoms


National Institute of Neurological Disorders and Stroke. (2020). Carpal tunnel syndrome fact sheet. National Institutes of Health, U.S. Department of Health and Human Services. https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Fact-Sheets/Carpal-Tunnel-Syndrome-Fact-Sheet


Science Reference Section. (2019). What is the strongest muscle in the human body? Library of Congress. https://www.loc.gov/everyday-mysteries/biology-and-human-anatomy/item/what-is-the-strongest-muscle-in-the-human-body/


The skeletal system forms the framework of the body. It is the body system composed of bones, cartilage, and ligaments. Each bone serves a particular function and varies in size, shape, and strength. Bones are weight-bearing structures in your body and can therefore change in thickness as you gain or lose weight. The skeletal system performs the following critical functions for the human body:


Media 6.1. The Skeletal System: Crash Course A&P #19 [Online video]. Copyright 2015 by CrashCourse.


The skeletal system includes all of the bones, cartilages, and ligaments of the body that support and give shape to the body and body structures. The skeleton consists of the bones of the body. For adults, there are 206 bones in the skeleton. Younger individuals have higher numbers of bones because some bones fuse together during childhood and adolescence to form an adult bone. The primary functions of the skeleton are to provide a rigid, internal structure that can support the weight of the body against the force of gravity, and to provide a structure upon which muscles can act to produce movements of the body.


In addition to providing for support and movements of the body, the skeleton has protective and storage functions. It protects the internal organs, including the brain, spinal cord, heart, lungs, and pelvic organs. The bones of the skeleton serve as the primary storage site for important minerals such as calcium and phosphate. The bone marrow found within bones stores fat and houses the blood-cell-producing tissue of the body.


The skeleton is subdivided into two major divisions: the axial and appendicular.


The axial skeleton forms the vertical, central axis of the body and includes all bones of the head, neck, chest, and back (see Figure 6.1). It serves to protect the brain, spinal cord, heart, and lungs. It also serves as the attachment site for muscles that move the head, neck, and back, and for muscles that act across the shoulder and hip joints to move their corresponding limbs.


The axial skeleton of the adult consists of 80 bones, including the skull, the vertebral column, and the thoracic cage. The skull is formed by 22 bones. Also associated with the head are an additional seven bones, including the hyoid bone and the ear ossicles (three small bones found in each middle ear). The vertebral column consists of 24 bones, each called a vertebra, plus the sacrum and coccyx. The thoracic cage includes the 12 pairs of ribs and the sternum, the flattened bone of the anterior chest.


The cranium or skull supports the face and protects the brain. It is subdivided into the bones of the skull and the bones of the face.


The vertebral column is also known as the spinal column or spine (see Figure 6.2). It consists of a sequence of vertebrae (singular = vertebra), each of which is separated and united by an intervertebral disc. Together, the vertebrae and intervertebral discs form the vertebral column. It is a flexible column that supports the head, neck, and body and allows for their movements. It also protects the spinal cord, which passes down the back through openings in the vertebrae.


The thoracic cage (rib cage) forms the thorax (chest) portion of the body. It consists of the 12 pairs of ribs with their costal cartilages and the sternum (see Figure 6.3). The ribs are anchored posteriorly to the 12 thoracic vertebrae (T1–T12). The thoracic cage protects the heart and lungs.


There are 12 sets of ribs and can be divided as such:


The sternum, also known as the breast bone, is divided into 3 parts:


The bones of the upper limbs include the bones of the arms, wrists, and hands.


Each phalanx has three bones: the distal, medial, and proximal. The exception is the thumb and big toe which has two bones: the distal and proximal (Figure 6.5). There are 30 bones in each upper limb. Can you count them on your limb?


The bones of the pelvic region protect the reproductive, urinary, and excretory organs.


The shape of the pelvic girdle is different for males than females. In the male, it is a funnel shape. In the female, it is shaped like a basin to accommodate the fetus during pregnancy.


The bones of the lower limb include bones of the leg and the feet.


The bones of the skeletal system are comprised of an inner spongy tissue referred to as bone marrow. There are two types of bone marrow: red and yellow. The red bone marrow produces the red blood cells, and it does so by a process called hematopoiesis. The yellow bone marrow contains adipose tissues which can be a source of energy. The bones of the skeletal system also store minerals such as calcium and phosphate. These minerals are important for the physiological processes in the body and are released into the bloodstream when levels are low in the body.


Media 6.2. Joints: Crash Course A&P #20 [Online video]. Copyright 2015 by CrashCourse.


Most bones connect to at least one other bone in the body. The area where bones meet bones or where bones meet cartilage are called articulations. Joints can be classified based on their ability to move. At movable joints, the articulating surfaces of the adjacent bones can move smoothly against each other. However, other joints may be connected by connective tissue or cartilage. These joints are designed for stability and provide for little or no movement. Importantly, joint stability and movement are related to each other. This means that stable joints allow for little or no mobility between the adjacent bones. Conversely, joints that provide the most movement between bones are the least stable.


Based on the function of joints, there are 3 types of joints:


Flexion and extension are movements that take place within the sagittal plane and involve anterior or posterior movements of the body or limbs. For the vertebral column, flexion (anterior flexion) is an anterior (forward) bending of the neck or body, while extension involves a posterior-directed motion, such as straightening from a flexed position or bending backward. Lateral flexion is the bending of the neck or body toward the right or left side. These movements of the vertebral column involve both the joints as well as the associated intervertebral disc.


In the limbs, flexion decreases the angle between the bones (bending of the joint), while extension increases the angle and straightens the joint (see Figures 6.8(a-d)). You will discover in the muscular system chapter that the associated muscles to these movements are flexor and extensor.


Abduction and adduction motions occur within the coronal plane and involve medial-lateral motions of the limbs, fingers, toes, or thumb. For example, abduction is raising the arm at the shoulder joint, moving it laterally away from the body, while adduction brings the arm down to the side of the body (see Figure 6.8(e)). In the muscular system chapter, you will discover that the associated muscles to these movements are the abductor and adductor.


Circumduction is the movement of a body region in a circular manner, in which one end of the body region being moved stays relatively stationary while the other end describes a circle. It involves the sequential combination of flexion, adduction, extension, and abduction at a joint (see Figure 6.8(e)).


Rotation can occur within the vertebral column, at a pivot joint, or at a ball-and-socket joint. Rotation of the neck or body is the twisting movement produced by the summation of the small rotational movements available between adjacent vertebrae. At a pivot joint, one bone rotates in relation to another bone.


Rotation can also occur at the ball-and-socket joints of the shoulder and hip. Here, the humerus and femur rotate around their long axis, which moves the anterior surface of the arm or thigh either toward or away from the midline of the body (see Figure 6.8(f)).


Supination and pronation are movements of the forearm. In the anatomical position, the upper limb is held next to the body with the palm facing forward. This is the supinated position of the forearm. In this position, the radius and ulna are parallel to each other. When the palm faces backward, the forearm is in the pronated position, and the radius and ulna form an X-shape.


Pronation is the movement that allows the palm to face backward while in supination the palm faces forward. It helps to remember that supination is the motion you use when scooping up soup with a spoon (see Figure 6.9(g)).


Dorsiflexion and plantar flexion are movements at the ankle joint, which is a hinge joint. Lifting the front of the foot, so that the top of the foot moves (upward) toward the anterior leg is dorsiflexion, while lifting the heel of the foot from the ground or pointing the toes downward is plantar flexion. These are the only movements available at the ankle joint (see Figure 6.9(h)).


Inversion and eversion are complex movements that involve the multiple plane joints among the tarsal bones of the posterior foot (intertarsal joints) and thus are not motions that take place at the ankle joint. Inversion is the turning of the foot to angle the bottom of the foot toward the midline, while eversion turns the bottom of the foot away from the midline. The foot has a greater range of inversion than eversion motion. These are important motions that help to stabilize the foot when walking or running on an uneven surface and aid in the quick side-to-side changes in direction used during active sports such as basketball, racquetball, or soccer (see Figure 6.9(i)).


Protraction and retraction are anterior-posterior movements of the scapula or mandible. Protraction of the scapula occurs when the shoulder is moved forward, as when pushing against something or throwing a ball. Retraction is the opposite motion, with the scapula being pulled posteriorly and medially, toward the vertebral column. For the mandible, protraction occurs when the lower jaw is pushed forward, to stick out the chin, while retraction pulls the lower jaw backward (see Figure 6.9(j)).


Depression and elevation are downward and upward movements of the scapula or mandible. The upward movement of the scapula and shoulder is elevation, while a downward movement is depression. These movements are used to shrug your shoulders. Similarly, elevation of the mandible is the upward movement of the lower jaw used to close the mouth or bite on something, and depression is the downward movement that produces the opening of the mouth (see Figure 6.9(k)).


Fibromyalgia is a challenging disease to diagnose since symptoms manifest differently and are similar to other diseases. Signs and symptoms may include widespread pain, chronic fatigue, gastrointestinal problems, and headaches. It is not known what causes fibromyalgia. A doctor may need to order tests to rule out other conditions before making a diagnosis of fibromyalgia (National Institute of Arthritis and Musculoskeletal and Skin Diseases, n.d.-c). To learn more about the diagnosis and treatment for fibromyalgia, please read this handout from the National Institute of Arthritis and Musculoskeletal and Skin Diseases (pdf).


Osteomyelitis is a bone infection caused when staphylococcus bacteria travel through the bloodstream from an infection in one part of the body to the bone. Staphylococcus bacteria are found on the skin, and they can transfer to the bone through a wound and/or surgical contamination. The risk increases as people age or if their immune system is compromised (Momodu & Savaliya, 2021). To learn more, visit the Mayo Clinic’s web page on osteomyelitis.


Developmental anomalies, pathological changes, or obesity can enhance the normal vertebral column curves, resulting in the development of abnormal or excessive curvatures (see Figure 6.10). Disorders associated with the curvature of the spine include:


A fracture is a broken bone. It will heal whether or not a physician resets it in its anatomical position. If the bone is not reset correctly, the healing process will keep the bone in its deformed position. Crepitation or crepitus is the creaking or popping sound that is heard when fractured bones move against each other. Fractures are classified by their complexity, location, and other features (see Figure 6.12). Some fractures may be described using more than one term because they may have the features of more than one type (e.g., an open transverse fracture).


There are three types of primary bone cancers: osteosarcoma, Ewing Sarcomas, and chondrosarcoma. These are considered primary cancers because they originate in the bones. Osteosarcoma and Ewing Sarcomas primarily affect children, teenagers, and young adults. Chondrosarcoma primarily affects older adults (National Cancer Institute, n.d.-a). To learn more, visit the American Cancer Society’s web page on bone cancer.


Orthopedic surgeons are medical doctors who have specialized training in the prevention, diagnosis, treatment, and surgery of disorders and diseases related to the musculoskeletal systems (Bureau of Labor Statistics, 2021a). For more details, please visit the American College of Surgeons’ page on Orthopedic Surgery.


Rheumatologists are medical doctors who specialize in the diagnosis and treatment of disorders of the joints, muscles, and bones. They diagnose and treat diseases such as arthritis, musculoskeletal disorders, osteoporosis, plus autoimmune diseases like ankylosing spondylitis, a chronic spinal inflammatory disease, and rheumatoid arthritis (Fowler et al., 2013). For more details, please follow the link to the American College of Rheumatology’s page on rheumatology.


Chiropractors are required to have a Doctor of Chiropractic (D.C.) degree, which is a 4-year postgraduate professional degree, and a state license. Chiropractors focus on spinal adjustments, nutrition, and preventing injury without the use of pharmaceuticals or surgical procedures (Bureau of Labor Statistics, 2021b). To learn more, visit the Bureau of Labor Statistics’ website.


A physical therapist is a licensed professional who develops individualized treatment plans for their clients. These plans can include exercises, hands-on therapy, and equipment, such as canes or wheelchairs. Although current licensure laws require that those entering the field have a doctor of physical therapy degree, physical therapists who began working before those requirements went into effect may have a bachelor’s or master’s degree (Bureau of Labor Statistics, 2021c). To learn more, please visit the American Physical Therapy Association website.


Common diagnostic procedures related specifically to the skeletal system include x-rays, bone mineral density testing, and arthroscopy.


Moving the limb or hand laterally away from the body, or spreading the fingers or toes.


Movement that brings the limb or hand toward or across the midline of the body, or brings the fingers or toes together.


All bones of the upper and lower limbs, plus the girdle bones that attach each limb to the axial skeleton.


Disorders in which the immune system overreacts and begins to attack itself.


The central, vertical axis of the body, including the skull, vertebral column, and thoracic cage.


An operation in which a piece of the skull is removed.


Foot movement in which the bottom of the foot is turned laterally, away from the midline.


Movement in the sagittal plane that increases the angle of a joint (straightens the joint).


Movement in the sagittal plane that decreases the angle of a joint (bends the joint).


Foot movement in which the bottom of the foot is turned toward the midline.


An excessive posterior curvature of the thoracic region; also called humpback.


Excessive anterior curvature of the lumbar vertebral column region; also called swayback.


Pertaining to the lumbar region of the spine (L1 to L5).


Pertaining to the region of the back that includes the lumbar vertebrae, sacrum, and nearby structures.


The most common type of arthritis; associated with aging and “wear and tear” of the articular cartilage.


A disease characterized by a decrease in bone mass that occurs when the rate of bone resorption exceeds the rate of bone formation.


Forearm motion that moves the palm of the hand from the palm forward to the palm backward position.


Movement of a bone around a central axis or around its long axis.


A degenerative spinal disease that can involve any part of the vertebra, intervertebral disk, and surrounding soft tissue.


Forearm motion that moves the palm of the hand from the palm backward to the palm forward position.


A procedure used to repair a bone in the spine that has a break caused by cancer, osteoporosis, or trauma.


Bureau of Labor Statistics. (2021b). Chiropractors. In Occupational outlook handbook. U.S. Department of Labor. https://www.bls.gov/ooh/healthcare/chiropractors.htm


Bureau of Labor Statistics. (2021c). Physical therapists. In Occupational outlook handbook. U.S. Department of Labor. https://www.bls.gov/ooh/healthcare/physical-therapists.htm


Centers for Disease Control and Prevention. (n.d.-a). Osteoarthritis. https://www.cdc.gov/arthritis/basics/osteoarthritis.htm


Centers for Disease Control and Prevention. (n.d.-b). Rheumatoid arthritis. CDC Arthritis Program. https://www.cdc.gov/arthritis/basics/rheumatoid-arthritis.html


Centers for Disease Control and Prevention. (n.d.-c). Gout. https://www.cdc.gov/arthritis/basics/gout.html


Fowler, S., Roush, R., & Wise, J. (2013). Concepts of biology. OpenStax. Access for free at https://openstax.org/books/concepts-biology/pages/1-introduction


MedlinePlus. (2021). X-rays. U.S. National Library of Medicine. https://medlineplus.gov/xrays.html


Momodu, I. I., & Savaliya, V. (2021). Osteomyelitis. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK532250/


National Cancer Institute. (n.d.-a). Primary bone cancer [Fact sheet]. National Institutes of Health, U.S. Department of Health and Human Services. https://www.cancer.gov/types/bone/bone-fact-sheet


National Cancer Institute. (n.d.-b). Dual x-ray absorptiometry. National Institutes of Health, U.S. Department of Health and Human Services. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/dual-x-ray-absorptiometry


National Institute of Arthritis and Musculoskeletal and Skin Diseases. (n.d.-a). Arthritis. National Institutes of Health, U.S. Department of Health and Human Services. https://www.niams.nih.gov/health-topics/arthritis


National Institute of Arthritis and Musculoskeletal and Skin Diseases. (n.d.-b). Osteoporosis overview. National Institutes of Health, U.S. Department of Health and Human Services. https://www.bones.nih.gov/health-info/bone/osteoporosis/overview


National Institutes of Arthritis and Musculoskeletal and Skin Diseases. (n.d-c). Health topics: Fibromyalgia [PDF]. National Institutes of Health, U.S. Department of Health and Human Services. https://www.niams.nih.gov/print/view/pdf/advanced_reading_pdf_/advanced?view_args%5B0%5D=1957


Office of Communications and Public Liaison. (2020). Myasthenia gravis fact sheet. National Institute of Neurological Disorders and Stroke, U.S. Department of Health and Human Services. https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Fact-Sheets/Myasthenia-Gravis-Fact-Sheet


The integumentary system refers to the skin and its accessory structures. In the adult human body, the skin makes up about 16% of body weight and covers an area of 1.5 to 2 m2.


In fact, the skin and accessory structures are the largest organ system in the human body. The skin protects your inner organs, and it is in need of daily care and protection to maintain its health.


Media 5.1. The Integumentary System, Part 1 – Skin Deep: Crash Course A&P #6 [Video]. Copyright 2015 by CrashCourse.


The epidermis is composed of keratinized, stratified squamous epithelium. It is made of four or five layers of epithelial cells, depending on its location in the body. It is avascular.


The cells in all of the layers except the stratum basale are called keratinocytes. Keratin is an intracellular fibrous protein that gives hair, nails, and skin their hardness and water-resistant properties. The keratinocytes in the stratum corneum are dead and regularly slough away, being replaced by cells from the deeper layers (see Figure 5.3).


The papillary layer is made of loose, areolar connective tissue, which means the collagen and elastin fibers of this layer form a loose mesh. This superficial layer of the dermis projects into the stratum basale of the epidermis to form finger-like dermal papillae (see Figure 5.4). Within the papillary layer are fibroblasts, a small number of adipocytes, and an abundance of small blood vessels. In addition, the papillary layer contains phagocytes that help fight bacteria or other infections that have breached the skin. The layer also contains lymphatic capillaries, nerve fibers, and Meissner corpuscles.


Underlying the papillary layer is the much thicker reticular layer, composed of dense, irregular connective tissue. The layer is well vascularized and has a rich sensory and sympathetic nerve supply. The reticular layer appears reticulated due to a tight meshwork of fibers. Elastin fibers provide some elasticity to the skin, enabling movement. Collagen fibers provide structure and tensile strength, with strands of collagen extending into both the papillary layer and the hypodermis. In addition, collagen binds water to keep the skin hydrated. Collagen injections and Retin-A creams help restore skin turgor by either introducing collagen externally or stimulating blood flow and repair of the dermis, respectively.


The hypodermis, also known as the subcutaneous layer, serves to connect the skin to the underlying fascia of the bones and muscles. It is not strictly a part of the skin, although the border between the hypodermis and dermis can be difficult to distinguish. The hypodermis consists of well-vascularized, loose, areolar connective tissue and adipose tissue, which functions as a mode of fat storage and provides insulation and cushioning for the integument.


The skin and accessory structures perform a variety of essential functions, such as protecting the body from invasion by microorganisms, chemicals, and other environmental factors; preventing dehydration; acting as a sensory organ; modulating body temperature and electrolyte balance; and synthesizing vitamin D. The underlying hypodermis has important roles in storing fats, forming a “cushion” over underlying structures, and providing insulation from cold temperatures.


The skin protects the body from wind, water, and UV sunlight. It acts as a protective barrier against water loss and it also is the first line of defense against abrasive activity such as grit, microbes, or harmful chemicals. Sweat excreted from sweat glands deters microbes from over-colonizing the skin surface by generating dermcidin, which has antibiotic properties.


The skin acts as a sense organ because the epidermis, dermis, and hypodermis contain specialized sensory nerve structures that detect touch, surface temperature, and pain. These receptors are more concentrated on the tips of the fingers, which are most sensitive to touch, especially the Meissner corpuscle, which responds to light touch, and the Pacinian corpuscle, which responds to vibration. Merkel cells, seen scattered in the stratum basale, are also touch receptors. In addition to these specialized receptors, there are sensory nerves connected to each hair follicle, pain and temperature receptors scattered throughout the skin, and motor nerves innervate the arrector pili muscles and glands. This rich innervation helps us sense our environment and react accordingly,


The integumentary system helps regulate body temperature through its tight association with the sympathetic nervous system. The sympathetic nervous system is continuously monitoring body temperature and initiating appropriate motor responses.


Media 5.2. The Integumentary System, Part 2 – Skin Deeper: Crash Course A&P #7 [Online video]. Copyright 2015 by CrashCourse.


Accessory structures of the skin include hair, nails, sweat glands, and sebaceous glands. These structures embryologically originate from the epidermis and can extend down through the dermis into the hypodermis.


Hair is a keratinous filament growing out of the epidermis. It is primarily made of dead, keratinized cells. Strands of hair originate in an epidermal penetration of the dermis called the hair follicle. The hair shaft is the part of the hair not anchored to the follicle, and much of this is exposed at the skin’s surface. The rest of the hair, which is anchored in the follicle, lies below the surface of the skin and is referred to as the hair root. The hair root ends deep in the dermis at the hair bulb and includes a layer of mitotically active basal cells called the hair matrix. The hair bulb surrounds the hair papilla, which is made of connective tissue and contains blood capillaries and nerve endings from the dermis (see Figure 5.6).


Hair also has a sensory function due to sensory innervation by a hair root plexus surrounding the base of each hair follicle. Hair is extremely sensitive to air movement or other disturbances in the environment, much more so than the skin surface. This feature is also useful for the detection of the presence of insects or other potentially damaging substances on the skin surface.


Each hair root is connected to a smooth muscle called the arrector pili that contracts in response to nerve signals from the sympathetic nervous system, making the external hair shaft “stand up.” The primary purpose for this is to trap a layer of air to add insulation. This is visible in humans as goosebumps. Of course, this is much more obvious in organisms with a heavier coat than most humans, such as when a frightened cat raises its fur.


Hair grows and is eventually shed and replaced by new hair. Hair typically grows at the rate of 0.3 mm per day. On average, 50 hairs are lost and replaced per day. Hair loss occurs if there is more hair shed than what is replaced and can happen due to hormonal or dietary changes. Hair loss can also result from the aging process, or the influence of hormones. Similar to the skin, hair gets its color from the pigment melanin, produced by melanocytes in the hair papilla. Different hair color results from differences in the type of melanin. As a person ages, the melanin production decreases, and hair tends to lose its color and becomes gray and/or white.


The nail bed is a specialized structure of the epidermis that is found at the tips of our fingers and toes. The nail body is formed on the nail bed and protects the tips of our fingers and toes as they are the farthest extremities and the parts of the body that experience the maximum mechanical stress (see Figure 5.7). The nail body forms a back-support for picking up small objects with the fingers. The nail body is composed of densely packed dead keratinocytes.


The epidermis in this part of the body has evolved a specialized structure upon which nails can form. The nail body forms at the nail root, which has a matrix of proliferating cells from the stratum basale that enables the nail to grow continuously. The lateral nail fold overlaps the nail on the sides, helping to anchor the nail body. The nail fold that meets the proximal end of the nail body forms the nail cuticle, also called the eponychium.


The nail bed is rich in blood vessels, making it appear pink, except at the base where a thick layer of epithelium over the nail matrix forms a crescent-shaped region called the lunula (the “little moon”). The area beneath the free edge of the nail, furthest from the cuticle, is called the hyponychium. It consists of a thickened layer of stratum corneum.


When the body becomes warm, sudoriferous glands produce sweat to cool the body. Sweat glands develop from epidermal projections into the dermis and are classified as merocrine glands; that is, the secretions are secreted by exocytosis through a duct without affecting the cells of the gland. There are two types of sweat glands, each secreting slightly different products.


An eccrine sweat gland is a type of gland that produces hypotonic sweat for thermoregulation as described previously. These glands are found all over the skin’s surface but are especially abundant on the palms of the hand, the soles of the feet, and the forehead (Figure 5.8). They are coiled glands lying deep in the dermis, with the duct rising up to a pore on the skin surface where the sweat is released. This type of sweat, released by exocytosis, is hypotonic and composed mostly of water, with some salt, antibodies, traces of metabolic waste, and dermcidin, an antimicrobial peptide. Eccrine glands are a primary component of thermoregulation in humans and thus help to maintain homeostasis.


A sebaceous gland is a type of oil gland that is found all over the body and helps to lubricate and waterproof the skin and hair. Most sebaceous glands are associated with hair follicles. They generate and excrete sebum, a mixture of lipids, onto the skin surface, thereby naturally lubricating the dry and dead layer of keratinized cells of the stratum corneum, keeping it pliable. The fatty acids of sebum also have antibacterial properties and prevent water loss from the skin in low-humidity environments. The secretion of sebum is stimulated by hormones, many of which do not become active until puberty. Thus, sebaceous glands are relatively inactive during childhood.


Many terms and phrases related to the integumentary system are abbreviated. Learn these common abbreviations by expanding the list below.


All systems in the body accumulate subtle and some not-so-subtle changes as a person ages. Among these changes are reductions in cell division, metabolic activity, blood circulation, hormonal levels, and muscle strength (see Figure 5.9). In the skin, these changes are reflected in decreased mitosis in the stratum basale, leading to a thinner epidermis. The dermis, which is responsible for the elasticity and resilience of the skin, exhibits a reduced ability to regenerate, which leads to slower wound healing. The hypodermis, with its fat stores, loses structure due to the reduction and redistribution of fat, which in turn contributes to the thinning and sagging of skin.


The accessory structures also have lowered activity, generating thinner hair and nails, and reduced amounts of sebum and sweat. A reduced sweating ability can cause some elderly to be intolerant to extreme heat. Other cells in the skin, such as melanocytes and dendritic cells, also become less active, leading to a paler skin tone and lowered immunity. Wrinkling of the skin occurs due to the breakdown of its structure, which results from decreased collagen and elastin production in the dermis, weakening of muscles lying under the skin, and the inability of the skin to retain adequate moisture.


The integumentary system is susceptible to a variety of diseases, disorders, and injuries. These range from annoying but relatively benign bacterial or fungal infections that are categorized as disorders, to skin cancer and severe burns, which can be fatal. In this section, you will learn several of the most common skin conditions.


Most cancers are identified by the organ or tissue in which the cancer originates. In general, cancers result from an accumulation of DNA mutations. These mutations can result in cell populations that do not die when they should and uncontrolled cell proliferation that leads to tumors. Although many tumors are benign, some metastasize. Cancers are characterized by their ability to metastasize. One common form of cancer is skin cancer.


Melanin synthesis peaks about 10 days after initial sun exposure, which is why pale-skinned individuals tend to suffer sunburns of the epidermis initially. Dark-skinned individuals can also get sunburns but are more protected than are pale-skinned individuals. Too much sun exposure can eventually lead to wrinkling due to the destruction of the cellular structure of the skin, and in severe cases, can cause sufficient DNA damage to result in skin cancer. When there is an irregular accumulation of melanocytes in the skin, freckles appear. Moles are larger masses of melanocytes, and although most are benign, they should be monitored for changes that might indicate the presence of cancer (see Figure 5.10).


Basal cell carcinoma is a form of cancer that affects the mitotically active stem cells in the stratum basale of the epidermis. It is the most common of all cancers that occur in the United States and is frequently found on the head, neck, arms, and back, which are the most susceptible to long-term sun exposure. Although UV rays are the main culprit, exposure to other agents, such as radiation and arsenic, can also lead to this type of cancer. Wounds on the skin due to open sores, tattoos, burns, et cetera may be predisposing factors. Basal cell carcinomas start in the stratum basale and usually spread along this boundary. At some point, they begin to grow toward the surface and become an uneven patch, bump, growth, or scar on the skin surface (see Figure 5.11). Like most cancers, basal cell carcinomas respond best to treatment when caught early. Treatment options include surgery, freezing (cryosurgery), and topical ointments.


Squamous cell carcinoma is cancer that affects the keratinocytes of the stratum spinosum and presents as lesions commonly found on the scalp, ears, and hands (see Figure 5.12). It is the second most common skin cancer. The American Cancer Society reports that two of 10 skin cancers are squamous cell carcinomas, and it is more aggressive than basal cell carcinoma. If not removed, these carcinomas can metastasize. Surgery and radiation are used to cure squamous cell carcinoma.


Melanoma is cancer characterized by the uncontrolled growth of melanocytes, the pigment-producing cells in the epidermis. Typically, a melanoma develops from a mole. It is the most fatal of all skin cancers, as it is highly metastatic and can be difficult to detect before it has spread to other organs. Melanomas usually appear as asymmetrical brown and black patches with uneven borders and a raised surface (see Figure 5.13). Treatment typically involves surgical excision and immunotherapy.


Some specialists cite the following additional signs for the most serious form, nodular melanoma:


Treatment of this disorder usually involves addressing the symptoms, such as limiting UV light exposure to the skin and eyes. In vitiligo, the melanocytes in certain areas lose their ability to produce melanin, possibly due to an autoimmune reaction. This leads to a loss of color in patches (see Figure 5.14). Neither albinism nor vitiligo directly affects the lifespan of an individual.


Other changes in the appearance of skin coloration can be indicative of diseases associated with other body systems.


Eczema is an allergic reaction that manifests as dry, itchy patches of skin that resemble rashes (see Figure 5.15). It may be accompanied by swelling of the skin, flaking, and in severe cases, bleeding. Symptoms are usually managed with moisturizers, corticosteroid creams, and immunosuppressants.


Acne is a skin disturbance that typically occurs on areas of the skin that are rich in sebaceous glands (face and back). It is most common along with the onset of puberty due to associated hormonal changes, but can also occur in infants and continue into adulthood. Hormones, such as androgens, stimulate the release of sebum. Overproduction and accumulation of sebum along with keratin can block hair follicles. This plug is initially white. The sebum, when oxidized by exposure to air, turns black. Acne results from infection by acne-causing bacteria (Propionibacterium and Staphylococcus), which can lead to redness and potential scarring due to the natural wound healing process (see Figure 5.16).


Tinea or dermatophytosis is often referred to as ringworm. Ringworm presents as a circular rash that is itchy and red and can be found on various parts of the body. It is referred to by the location that it is found:


To learn more about ringworm, visit the Center for Disease Control and Prevention’s web page on fungal infections.


Psoriasis is a chronic autoimmune disorder that results in patches of thick red skin with the appearance of silvery scales. These patches can be found on elbows, knees, scalp, low back, face, feet, fingernails, toenails, and even the mouth. Psoriasis can be confused with other skin diseases, so a dermatologist is the best physician to diagnose psoriasis. Treatments may include creams, ointments, ultraviolet light therapy, and medication (Center for Disease Control and Prevention, n.d.-b). To learn more, visit the Center for Disease Control and Prevention’s web page on psoriasis.


Skin injuries set off a healing process that occurs in several overlapping stages.


A burn results when the skin is damaged by intense heat, radiation, electricity, or chemicals. The damage results in the death of skin cells, which can lead to a massive loss of fluid. Dehydration, electrolyte imbalance, and renal and circulatory failure follow, which can be fatal. Burn patients are treated with  intravenous fluids to offset dehydration, as well as intravenous nutrients that enable the body to repair tissues and replace lost proteins. Another serious threat to the lives of burn patients is infection. Burned skin is extremely susceptible to bacteria and other pathogens due to the loss of protection by intact layers of skin.


Burns are sometimes measured in terms of the size of the total surface area affected. This is referred to as the rule of nines, which associates specific anatomical areas with a percentage that is a factor of nine (see Figure 5.17).


Oddly, third and fourth-degree burns are usually not as painful because the nerve endings themselves are damaged. Full-thickness burns cannot be repaired by the body, because the local tissues used for repair are damaged and require debridement, or amputation in severe cases, followed by grafting of the skin from an unaffected part of the body, or from skin grown in tissue culture for grafting purposes. Skin grafts are required when the damage from trauma or infection cannot be closed with sutures or staples.


Most cuts or wounds, with the exception of ones that only scratch the epidermis, lead to scar formation. Scarring occurs in cases in which there is repair of skin damage, but the skin fails to regenerate the original skin structure. Fibroblasts generate scar tissue in the form of collagen, and the bulk of repair is due to the basket-weave pattern generated by collagen fibers and does not result in regeneration of the typical cellular structure of skin. Instead, the tissue is fibrous in nature and does not allow for the regeneration of accessory structures, such as hair follicles, sweat glands, or sebaceous glands.


Sometimes, there is an overproduction of scar tissue, because the process of collagen formation does not stop when the wound is healed; this results in a keloid. In contrast, scars that result from acne and chickenpox have a sunken appearance and are called atrophic scars.


Scarring of skin after wound healing is a natural process and does not need to be treated further. Application of mineral oil and lotions may reduce the formation of scar tissue. However, modern cosmetic procedures, such as dermabrasion, laser treatments, and filler injections have been invented as remedies for severe scarring. All of these procedures try to reorganize the structure of the epidermis and underlying collagen tissue to make it look more natural.


Skin and its underlying tissue can be affected by excessive pressure. One example of this is called a bedsore. Bedsores, also called decubitus ulcers, are caused by constant, long-term, unrelieved pressure on certain body parts that are bony, reducing blood flow to the area and leading to necrosis. Bedsores are most common in elderly patients who have debilitating conditions that cause them to be immobile. Most hospitals and long-term care facilities have the practice of turning the patients every few hours to prevent the incidence of bedsores. If left untreated, bedsores can be fatal if they become infected.


The skin can also be affected by pressure associated with rapid growth. A stretch mark results when the dermis is stretched beyond its limits of elasticity, as the skin stretches to accommodate the excess pressure. Stretch marks usually accompany rapid weight gain during puberty and pregnancy. They initially have a reddish hue but lighten over time. Other than for cosmetic reasons, treatment of stretch marks is not required. They occur most commonly over the hips and abdomen.


When you wear shoes that do not fit well and are a constant source of abrasion on your toes, you tend to form a callus at the point of contact. This occurs because the basal stem cells in the stratum basale are triggered to divide more often to increase the thickness of the skin at the point of abrasion to protect the rest of the body from further damage. This is an example of a minor or local injury, and the skin manages to react and treat the problem independently of the rest of the body. Calluses can also form on your fingers if they are subject to constant mechanical stress, such as long periods of writing, playing string instruments, or video games. A corn is a specialized form of callus. Corns form from abrasions on the skin that result from an elliptical-type motion.


A dermatologist is a medical doctor with specialized training in treating diseases, disorders, and injuries related to the integumentary system and its accessory structures. There are many dermatologic subspecialties, such as cosmetic dermatology, dermatopathology, and pediatric dermatology. To learn more, visit the American Academy of Dermatology Association’s webpage on dermatology as a career.


Dermatologists can be specially trained to perform a procedure called Mohs surgery. Mohs surgery excises skin cancers in thin layers until all cancer is removed from the tissue (Prickett & Ramsey, 2021).


An enclosed collection of pus in tissues, organs, or confined spaces in the body.


Single-cell microorganisms that reproduce by cell division and may cause infection by invading body tissue.


The removal of cells or tissues for examination by a pathologist.


To destroy tissue using a hot or cold instrument, an electrical current, or a chemical that burns or dissolves the tissue to kill tumors or stop bleeding.


An infection of the skin and subcutaneous tissue, characterized by tenderness, fever, and blisters.


Excision of damaged tissues and cell debris from a wound or burn to prevent infection and promote healing.


A net loss of water that results in insufficient water in blood and other tissues.


A procedure to remove superficial scars using sandpaper or revolving wire brushes.


The layer of skin that is made of dense, irregular connective tissue that houses blood vessels, hair follicles, sweat glands, and other structures.


A surgical procedure used to remove moles, cysts, skin cancer, and other skin growths using local anesthesia.


A form of active transport in which a cell exports material using vesicular transport.


A condition in which conservation of the body core heat results in the skin freezing.


Also known as the subcutaneous layer; the layer of the skin below the dermis that is composed mainly of loose connective and fatty tissues.


The invasion and growth of bacteria, viruses, yeast, fungi, or other microorganisms in the body.


The process in which cancer spreads from one part of the body to another.


A benign growth on the skin that is formed by a cluster of melanocytes.


Passing through the skin, as an injection or a topical medicine.


A painful condition in children where bones are misshapen due to a lack of calcium, causing bow-leggedness.


A collagen-rich skin formed after the process of wound healing that differs from normal skin.


A bacteria that is commonly found in minor skin infections, as well as in the nose of some healthy people.


A group of fungal skin diseases of the hair, skin, and nail tissues.


A simple microorganism that may cause infection by invading body tissue.


Centers for Disease Control and Prevention. (n.d.-b). Psoriasis. Centers for Disease Control and Prevention: Fungal Diseases. https://www.cdc.gov/psoriasis/


CrashCourse. (2015, January 6). The Integumentary system, part 1 – skin deep: Crash Course A&P #6 [Video]. YouTube. https://youtu.be/Orumw-PyNjw


CrashCourse. (2015, February 16). The Integumentary system, part 2 – skin deeper: Crash Course A&P #7 [Video]. YouTube. https://youtu.be/EN-x-zXXVwQ


Prickett, K. A., & Ramsey, M. L. (2021). Mohs micrographic surgery. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK441833/


Ask anyone what the senses are, and they are likely to list the five major senses as taste, smell, touch, hearing, and sight. However, these are not all of the senses. The most obvious omission from this list is balance. Touch can be further subdivided into pressure, vibration, stretch, and hair-follicle position based on the type of mechanoreceptors that perceive these touch sensations. Other overlooked senses include temperature perception by thermoreceptors and pain perception by nociceptors.


Within the realm of physiology, senses can be classified as either general or special. A general sense is one that is distributed throughout the body and has receptor cells within the structures of other organs. Mechanoreceptors in the skin, muscles, or walls of blood vessels are examples of this type. General senses often contribute to the sense of touch, as described above, or to proprioception and kinesthesia, or to a visceral sense, which is most important to autonomic functions. A special sense is one that has a specific organ devoted to it, namely the eye, inner ear, tongue, or nose.


Media 4.1 Taste & Smell: Crash Course A&P #16 [Online video]. Copyright 2015 from CrashCourse.


Gustation is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. Raised bumps called papillae contain the structures for gustatory transduction. There are four types of papillae, based on their appearance:


Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. Neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves.


Only a few recognized submodalities exist within the sense of taste, or gustation. Until recently, only four tastes were recognized: sweet, salty, sour, and bitter. Research at the turn of the 20th century led to the recognition of the fifth taste, umami, during the mid-1980s. Very recent research has suggested that there may also be a sixth taste for fats, or lipids.


Media 4.2 Hearing & Balance: Crash Course A&P #17 [Online video]. Copyright 2015 by CrashCourse.


Hearing, or audition, is the transduction of sound waves into a neural signal that is made possible by the structures of the ear (see Figure 4.1).


The image below is a cross-sectional view of the cochlea that shows the scala vestibuli and scala tympani run along both sides of the cochlear duct (Figure 4.2). The cochlear duct contains several organs of Corti, which transduce the wave motion of the two scalas into neural signals. The organs of Corti lie on top of the basilar membrane, which is the side of the cochlear duct located between the organs of Corti and the scala tympani. As the fluid waves move through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot, depending on the frequency of the waves. Higher frequency waves move the region of the basilar membrane that is close to the base of the cochlea. Lower frequency waves move the region of the basilar membrane that is near the tip of the cochlea.


Somatosensation is considered a general sense, as opposed to the special senses discussed in this section. Somatosensation is the group of sensory modalities that are associated with touch, proprioception, and interoception. These modalities include pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception, and kinesthesia. This means that its receptors are not associated with a specialized organ, but are instead spread throughout the body in a variety of organs. Many of the somatosensory receptors are located in the skin, but receptors are also found in muscles, tendons, joint capsules, ligaments, and in the walls of visceral organs.


The two types of somatosensory signals that are transduced by free nerve endings are pain and temperature. Temperature receptors are stimulated when local temperatures differ from body temperature. Some thermoreceptors are sensitive to just cold and others to just heat. Nociception is the sensation of potentially damaging stimuli. Mechanical, chemical, or thermal stimuli beyond a set threshold will elicit painful sensations. Stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors.


For example, the sensation of heat associated with spicy foods involves capsaicin, the active molecule in hot peppers. Capsaicin molecules bind to a transmembrane ion channel in nociceptors that is sensitive to temperatures above 37°C (98.6°F). The dynamics of capsaicin binding with this transmembrane ion channel are unusual in that the molecule remains bound for a long time. Because of this, it will decrease the ability of other stimuli to elicit pain sensations through the activated nociceptor. For this reason, capsaicin can be used as a topical analgesic, such as in products such as Icy Hot™.


Media 4.3 Vision: Crash Course A&P #18 [Online video]. Copyright 2015 by CrashCourse.


Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye (see Figure 4.4). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye.


The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the sclera, connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located beneath the lateral edges of the nose. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye where the tears flow over the conjunctiva, washing away foreign particles.


Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball. Four of the muscles are arranged at the cardinal points around the eye and are named for those locations. They are the:


When each of these muscles contracts, the eye moves toward the contracting muscle. For example, when the superior rectus contracts, the eye rotates to look up.


The eye itself is a hollow sphere composed of three layers of tissue:


The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors (rods and cones) change their membrane potential when stimulated by light energy. The change in membrane potential alters the number of neurotransmitters that the photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion cell (RGC) in the inner synaptic layer. There, amacrine cells additionally contribute to retinal processing before an action potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and leave the eye at the optic nerve (see Figure 4.5). Because these axons pass through the retina, there are no photoreceptors at the very back of the eye where the optic nerve begins. This creates a “blind spot” in the retina and a corresponding blind spot in our visual field.


Photoreceptors in the retina (rods and cones) are located behind the axons, RGCs, bipolar cells, and retinal blood vessels. A significant amount of light is absorbed by these structures before the light reaches the photoreceptor cells. At the exact center of the retina is a small area known as the fovea. At the fovea, the retina lacks the supporting cells and blood vessels, and only contains photoreceptors. Therefore, visual acuity is greatest at the fovea. This is because the fovea is where the least amount of incoming light is absorbed by other retinal structures (see Figure 4.5). As one moves in either direction from this central point of the retina, visual acuity drops significantly.


There are three types of cone opsins that are sensitive to different wavelengths of light and provide us with color vision. By comparing the activity of the three different cones, the brain can extract color information from visual stimuli (see Figure 4.6). For example, a bright blue light that has a wavelength of approximately 450 nm would activate the “red” cones minimally, the “green” cones marginally, and the “blue” cones predominantly. The relative activation of the three different cones is calculated by the brain, which perceives the color as blue. However, cones cannot react to low-intensity light, and rods do not sense the color of light. Therefore, our low-light vision is, in essence, in grayscale. In other words, in a dark room, everything appears as a shade of gray. If you think that you can see colors in the dark, it is most likely because your brain knows what color something is and is relying on that memory.


Blunt force trauma to the face, such as that common in many car accidents, can lead to the loss of the olfactory nerve, and subsequently, loss of the sense of smell. This condition is known as anosmia. When the frontal lobe of the brain moves relative to the ethmoid bone, the olfactory tract axons may be sheared apart. Professional fighters often experience anosmia because of repeated trauma to the face and head. In addition, certain pharmaceuticals, such as antibiotics, can cause anosmia by killing all the olfactory neurons at once. If no axons are in place within the olfactory nerve, then the axons from newly formed olfactory neurons have no guide to lead them to their connections within the olfactory bulb. There are temporary causes of anosmia, as well, such as those caused by inflammatory responses related to respiratory infections or allergies.


Loss of the sense of smell can result in food tasting bland. A person with an impaired sense of smell may require additional spice and seasoning levels for food to be tasted. Anosmia may also be related to some presentations of mild depression, because the loss of enjoyment of food may lead to a general sense of despair. The ability of olfactory neurons to replace themselves decreases with age, leading to age-related anosmia. This explains why some older adults salt their food more than younger people do. However, this increased sodium intake can increase blood volume and blood pressure, increasing the risk of cardiovascular diseases in older adults.


Otitis Media is known as inflammation of the middle ear canal that involves the eardrum. It is commonly seen in younger children due to bacterial and viral infections. Signs and symptoms include fever, cough and cold symptoms, hearing loss, irritability, and otalgia. Treatment involves symptomatic control as well as antibiotic (amoxicillin) use if necessary (Centers for Disease Control and Prevention, n.d.-a; MedlinePlus, 2021).


Otitis Externa is inflammation of the external ear canal. It is known as swimmer’s ear because it is associated with its exposure to water. Its clinical presentation and management are the same as otitis media (Centers for Disease Control and Prevention, n.d.-b).


Conductive hearing loss occurs when something disrupts sound through the mid and outer ear, such as physical damage to the eardrum (perforation). Hearing loss can be managed with pharmacotherapy, surgery, or a combination of the two (National Institute on Aging, n.d.).


Sensorineural hearing loss results from damage to the neural structures. This type of hearing loss is usually permanent (National Institute on Aging, n.d.).


Tinnitus is a condition in which a person hears ringing, roaring, or buzzing in their ears. It may be a symptom of earwax blocking the ear canal, hearing loss, hormonal changes in women, or a brain tumor. Although there is no cure, several treatments are available (National Institute on Deafness and Other Communication Disorders, 2017).


Otosclerosis is the hardening of the ear due to new bone formation of the inner ear ossicles. Its etiology may be related to prior measles infection, stress fractures to the tissue surrounding the inner ear, or immune disorders. Mild otosclerosis can be surgically treated (National Institute on Deafness and Other Communication Disorders, 2018).


Rhinitis is inflammation of the nasal cavity mucosal lining which can lead to congestion and rhinorrhea. Potential causes include allergy, bacterial or viral infection, and exposure to chemicals. Treatment regimens include symptom management, saline sprays, and oral antihistamines (Akhouri & House, 2021).


Dacryostenosis, also known as nasolacrimal obstruction, is an obstruction of the nasolacrimal duct. It prevents tears from draining from the eyes into the ducts and, thus, individuals have excessive tearing. Its etiology is congenital and the result of the duct not forming properly. This condition is managed via observation, as it resolves over time (usually within 1 year) (Pezzoli & Patel, 2021).


The term “blindness” may cover a broad spectrum of visual disability, from limited visibility to total blindness. The parameters for legal blindness are visual field is 20 degrees or narrower and/or visual acuity is 20/200 or less in both eyes even after correction (Lee & Mesfin, 2021). Many of the conditions described below lead to visual disability, low vision, and legal blindness.


Conjunctivitis is a condition involving inflammation of the conjunctiva. Common causes include allergens and bacterial and viral pathogens. The cause of conjunctivitis determines if it is transmissible from one individual to another; conjunctivitis caused by the adenovirus, for example, is highly contagious, whereas conjunctivitis caused by pollen is not. Management involves treating the underlying cause of conjunctivitis (Ryder & Benson, 2021).


Glaucoma is a condition in which increased pressure in the eye leads to progressive vision loss. It is the second most common cause of permanent blindness in the United States. The most common form of glaucoma is primary open-angle glaucoma. This form is associated with elevated pressure caused by a backup of fluid in the eye. Management depends on the type of glaucoma and the severity of the case. Glaucoma-related vision loss cannot be reversed (Dietze et al., 2021).


Macular degeneration is the progressive damage of a portion of the retina known as the macula. Severe central vision is lost with peripheral vision retained. Macular degeneration is the leading cause of blindness in people over the age of 60 years (Ruia & Kaufman, 2021).


Retinal detachment occurs when the retina gets pulled away or separated from its normal position. Signs and symptoms include flashing lights, floaters, and vision loss. Treatment generally involves surgery. If left untreated, a retinal detachment can lead to blindness (Blair & Czyz, 2021).


Strabismus is a condition where the affected eye rotates due to mismatched eye coordination. Each eye is focused differently as described below:


If not managed, the brain may reject input from one eye, resulting in amblyopia. Also known as lazy eye, amblyopia is caused when there is an imbalance of stimuli from the brain to the eyes (one eye receives more than the other). It usually occurs in childhood and requires early intervention to rectify this condition (New York State Department of Health, 2012).


A benign tumor of the vestibular nerve in the internal auditory canal.


A condition in which the lens of the eye becomes cloudy.


Creation of an artificial opening between the lacrimal sac and the nose (to restore drainage).


Sense of body movement based on sensation in the skeletal muscles, tendons, joints, and the skin.


A doctor who has special training in diagnosing and treating eye problems.


A surgical specialty focused on the structure, function, and surgery of the eye.


An exam of the fundus of the eye using a magnifying lens and light.


A specialist who diagnoses, treats, and manages diseases and disorders of the eye.


The professional practice of eye and vision care that involves measuring vision.


A doctor who has special training in diagnosing and treating diseases of the ear, nose, and throat.


A condition in which the eyes are more sensitive than normal to light.


Akhouri, S., & House, S. A. (2021). Allergic rhinitis. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK538186/


Blair, K., & Czyz, C. N. (2021). Retinal detachment. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK551502/


Centers for Disease Control and Prevention. (n.d.-a). Ear infection. https://www.cdc.gov/antibiotic-use/ear-infection.html


Centers for Disease Control and Prevention. (n.d.-b). Swimming and ear infections. https://www.cdc.gov/healthywater/swimming/swimmers/rwi/ear-infections.html


CrashCourse. (2015, May 11). Vision: Crash course A&P #18 [Video]. YouTube. https://www.youtube.com/watch?v=o0DYP-u1rNM


CrashCourse. (2015, April 27). Taste & smell: Crash course A&P #16 [Video]. YouTube. https://www.youtube.com/watch?v=mFm3yA1nslE


CrashCourse. (2015, May 4). Hearing & balance: Crash course A&P #17 [Video]. YouTube. https://www.youtube.com/watch?v=Ie2j7GpC4JU


Dietze, J., Blair, K., Havens, S. J. (2021). Glaucoma. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK538217/


Lee, S. Y., & Mesfin, F. B. (2021). Blindness. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK448182/


MedlinePlus. (2021). Ear infections. National Institutes of Health, U.S. Department of Health and Human Services. https://medlineplus.gov/earinfections.html


National Institute on Aging. (n.d.). Hearing loss: A common problem for older adults. National Institutes of Health, U.S. Department of Health and Human Services. https://www.nia.nih.gov/health/hearing-loss-common-problem-older-adults


National Institute on Deafness and Other Communication Disorders. (2017). Tinnitus. National Institutes of Health, U.S. Department of Health and Human Services. https://www.nidcd.nih.gov/health/tinnitus


National Institute on Deafness and Other Communication Disorders. (2018). Otosclerosis. National Institutes of Health, U.S. Department of Health and Human Services. https://www.nidcd.nih.gov/health/otosclerosis


New York State Department of Health. (2012). Types of vision problems. https://www.health.ny.gov/diseases/conditions/vision_and_eye_health/types_of_vision_problems.htm


Nizami, A. A., & Gulani, A. C. (2021). Cataract. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK539699/


Pezzoli, M., & Patel, B. C. (2021). Dacryostenosis. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK563132/


Ruia, S., & Kaufman, E. J. (2021). Macular degeneration. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK560778/


Ryder, E. C., & Benson, S. (2021). Conjunctivitis. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK541034/


Sekhon, R. K., Rocha Cabrero, F., & Deibel, J. P. (2020). Nystagmus types. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK539711/


Shukla, U. V., & Tripathy, K. (2021). Diabetic retinopathy. In StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK560805/


Anatomy focuses on structure and physiology focuses on function. Much of the study of physiology centers on the body’s tendency toward homeostasis.


Consider the structures of the body in terms of fundamental levels of organization that increase in complexity: subatomic particles, atoms, molecules, organelles, cells, tissues, organs, organ systems, organisms, and biosphere (Figure 3.1).


An organ system is a group of organs that work together to perform major functions or meet the physiological needs of the body.


is a group of organs that work together to perform major functions or meet the physiological needs of the body


Consider the breakdown into eleven distinct organ systems of the human body (Figure 3.2 and Figure 3.3). Assigning organs to organ systems can be imprecise since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system.


The organism level is the highest level of organization. An organism is a living being that has a cellular structure and that can independently perform all physiologic functions necessary for life. In multicellular organisms, including humans, all cells, tissues, organs, and organ systems of the body work together to maintain the life and health of the organism.


Media 3.1. Introduction to Anatomy & Physiology: Crash Course A&P #1 [Online video]. Copyright 2015 by CrashCourse.


Anatomists and healthcare providers use terminology for the purpose of precision and to reduce medical errors. For example, is a scar “above the wrist” located on the forearm two or three inches away from the hand? Or is it at the base of the hand? Is it on the palm-side or back-side? By using precise anatomical terminology, we eliminate ambiguity. Anatomical terms derive from ancient Greek and Latin words.


To further increase precision, anatomists standardize the way in which they view the body. Just as maps are normally oriented with north at the top, the standard body “map,” also known as the anatomical position, is that of the body standing upright with the feet at shoulder width and parallel, toes forward. The upper limbs are held out to each side, and the palms of the hands face forward as illustrated in Figure 3.4.


Using this standard position reduces confusion. It does not matter how the body being described is oriented, the terms are used as if it is in anatomical position. For example, a scar in the “anterior (front) carpal (wrist) region” would be present on the palm side of the wrist. The term “anterior” would be used even if the hand were palm down on a table.


A body that is lying down is described as either prone or supine. These terms are sometimes used in describing the position of the body during specific physical examinations or surgical procedures.


The human body’s numerous regions have specific terms to help increase precision. Notice that the term “brachium” or “arm” is reserved for the “upper arm” and “antebrachium” or “forearm” is used rather than “lower arm.” Similarly, “femur” or “thigh” is correct, and “leg” or “crus” is reserved for the portion of the lower limb between the knee and the ankle. You will be able to describe the body’s regions using the terms from the anatomical position.


Directional terms are essential for describing the relative locations of different body structures (Figure 3.5). For instance, an anatomist might describe one band of tissue as “inferior to” another or a physician might describe a tumor as “superficial to” a deeper body structure. Commit these terms to memory to avoid confusion when you are studying or describing the locations of particular body parts.


A section is a two-dimensional surface of a three-dimensional structure that has been cut. Modern medical imaging devices enable clinicians to obtain “virtual sections” of living bodies. We call these scans. Body sections and scans can be correctly interpreted, however, only if the viewer understands the plane along which the section was made. A plane is an imaginary two-dimensional surface that passes through the body. There are three planes commonly referred to in anatomy and medicine:


The body maintains its internal organization by means of membranes, sheaths, and other structures that separate compartments. The dorsal (posterior) cavity and the ventral (anterior) cavity are the largest body compartments (Figure 3.6). These cavities contain and protect delicate internal organs, and the ventral cavity allows for significant changes in the size and shape of the organs as they perform their functions. The lungs, heart, stomach, and intestines, for example, can expand and contract without distorting other tissues or disrupting the activity of nearby organs.


The posterior (dorsal) and anterior (ventral) cavities are each subdivided into smaller cavities.


To promote clear communication, for instance about the location of a patient’s abdominal pain or a suspicious mass, healthcare providers typically divide up the cavity into either nine regions or four quadrants.


A tissue membrane is a thin layer or sheet of cells that covers the outside of the body (for example, skin), the organs (for example, pericardium), internal passageways that lead to the exterior of the body (for example, abdominal mesenteries), and the lining of the movable joint cavities. There are two basic types of tissue membranes: connective tissue and epithelial membranes (Figure 3.7).


A synovial membrane is a type of connective tissue membrane that lines the cavity of a freely movable joint.


The epithelial membrane is composed of epithelium attached to a layer of connective tissue.


The mucous membrane is also a composite of connective and epithelial tissues.


The skin is an epithelial membrane also called the cutaneous membrane.


A serous membrane (also referred to as serosa) is an epithelial membrane composed of mesodermally derived epithelium called the mesothelium that is supported by connective tissue (Figure 3.8). These membranes line the coelomic cavities of the body and they cover the organs located within those cavities. They are essentially membranous bags, with mesothelial lining the inside and connective tissue on the outside.


There are three serous cavities and their associated membranes. Serous membranes provide additional protection to the viscera they enclose by reducing friction that could lead to inflammation of the organs.


That of the body standing upright, with the feet at shoulder width and parallel, toes forward. The upper limbs are held out to each side, and the palms of the hands face forward.


Describes the front or direction toward the front of the body.


A colorless fluid produced by the brain that cushions the brain and spinal cord within the posterior (dorsal) cavity.


Type of tissue that serves to hold in place, connect, and integrate the body’s organs and systems.


Epithelial tissue made up of stratified squamous epithelial cells that cover the outside of the body; skin.


Describes the middle or direction toward the middle of the body.


Epithelial membranes that line the body cavities and hollow passageways that open to the external environment.


Outermost layer of the pleura that connects to the thoracic wall, mediastinum, and diaphragm.