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Clinical Anatomy and Physiology for Veterinary Technicians Chapter 1

Anatomy and physiology

Author: Eze-Odikwa Tochukwu Jed

Note: All articles posted here are accurate, up-to-date and drafted from real university curriculums. Proper references will be added at the bottom of this article upon its completion. 

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College Reg Number: MOUAU/CME/14/18475

Anatomy and physiology describe two complementary but different ways to look at the animal body. Anatomy deals with the form and structure of the body and its parts—what things look like and where they are located. Physiology deals with the functions of the body and its parts—how things work and what they do. They can be studied as separate subjects, but such an approach makes it difficult to gain a complete picture of how the amazing animal machine works. This book examines anatomy and physiology together as we go along. We can approach the study of anatomy in different ways, for example microscopic anatomy versus macroscopic anatomy.

Microscopic anatomy deals with structures so small we need a microscope to see them clearly, such as cells and tissues.

Macroscopic anatomy, also called gross anatomy, deals with body parts large enough to be seen with the unaided eye, such as organs, muscles, and bones. Both aspects are presented in this book as we examine the animal body in detail. We also delve into the submicroscopic level occasionally to explain things occurring at the microscopic and macroscopic levels. Discussions at the submicroscopic level include the components that make up cells and the chemical molecules and ions that serve important roles in the body. Another way to approach anatomy is to study individual regions of the body (regional anatomy) versus individual systems of the body (systemic anatomy). In the regional approach, all the components of each region of the body are examined; for example, the anatomy of the neck (cervical) region would include all the cells, tissues, blood vessels, nerves, muscles, organs, and bones present in the neck. The problem is that the body is not always easy to subdivide this way, and there is often overlap between adjacent regions; that is, where does the neck region end and the shoulder region begin? It’s not always clear.

The systematic approach to anatomy, however, deals with the systems of the body, such as the nervous system and the skeletal system, as separate topics. The many interrelationships between the body systems can be described as the systems are examined. This approach lets us look at the whole body by breaking it down into clear, logical components. The main systems of the body are listed in Table 1-1. We will take a systematic approach to anatomy and physiology in this book, and in addition to these systems, we examine cells, epithelial and connective tissues, and blood, lymph, and immunity.

Table 1.1 Main Body Systems

SkeletalBones and joints
IntegumentarySkin, hair, nails, and hooves
NervousCentral nervous system and peripheral nerves
CardiovascularHeart and blood vessels
RespiratoryLungs and air passageways
DigestiveGastrointestinal tube and accessory digestive organs.
MuscularSkeletal, cardiac, and smooth muscle
SensoryOrgans of general and special sense
EndocrineEndocrine glands and hormones
UrinaryKidneys, ureters, urinary bladder, and urethra
ReproductiveMale and female reproductive structures


To be clear and accurate with descriptions of body parts, we have to use terms that leave no doubt about their meanings. Terms such as up, down, above, below, and beside are not very useful because they depend on the orientation of the animal (upright, on its side, on its back, and so on). If an animal is lying on its left side, is its right lung above its left lung or beside it? If the animal stands up, what is the relationship between the lungs then? Even the position of the observer can make a difference in terms such as left and right. If a structure in an animal is located “to the right” of another structure, does the meaning change if the observer is facing the animal head-on or facing the same direction as the animal? Anatomic terms must have the same meaning regardless of the orientation of the animal or the position of the observer. Basic anatomic terminology is based on imaginary slices, called planes, through the animal body that can be used as points or areas of reference and on sets of directional terms that have opposite meanings from each other.

Anatomy Planes of Reference

There are four anatomic planes of reference, two of which are variations of each other. Each plane is an imaginary slice through the body that is oriented at right angles to the other two. The four reference planes (Figure 1-1) are as follows:

Sagittal plane: A plane that runs the length of the body and divides it into left and right parts that are not necessarily equal halves.

Median plane: A special kind of sagittal plane that runs down the center of the body lengthwise and divides it into equal left and right halves. It could also be called a mid-sagittal plane, but that term is not commonly used.

Transverse plane: A plane across the body that divides it into cranial (head-end) and caudal (tail-end) parts that are not necessarily equal.

Dorsal plane: A plane at right angles to the sagittal and transverse planes. It divides the body into dorsal (toward the animal’s back) and ventral (toward the belly) parts that are not necessarily equal. If an animal stands in water with its body partially submerged, the surface of the water describes a dorsal plane. In humans this plane is called the frontal plane (Figure 1-2).

FIGURE 1-1 Anatomic planes of reference and directional terms. (Modified from McBride DF: Learning veterinary terminology, ed 2, St Louis, 2002, Mosby.)

FIGURE 1-2 Directions and planes of the human body. (From Thibodeau GA, Patton KT: Anatomy and physiology, ed 8, St Louis, 2013, Mosby.

Directional Terms

Directional terms in anatomy provide a common language for accurately and clearly describing body structures regardless of the position of the animal’s body. These terms generally occur in pairs that have opposite meanings and are used chiefly to describe relative positions of body parts. Since humans walk upright, there are a few differences between human directional terms and those of nonhuman animals (Table 1-2).

Left and right always refer to the animal’s left and right sides. The spleen, an organ with several important functions, is located on the left side of a cow’s abdomen. The duodenum, the first short portion of the small intestine, exits the stomach on the right side of a dog’s abdomen.

Cranial and caudal refer to the ends of the animal as it stands on four legs. Cranial means toward the head (cranium), and caudal means toward the tail (cauda). A horse’s shoulder is located cranial to its hip. The caudal end of the sternum (breastbone) is called the xiphoid process. In humans superior is used in place of cranial, and inferior is used in place of caudal.

Rostral is a special term used only to describe positions or directions on the head. The term cranial loses its meaning on the head because the cranium is part of the head. Caudal retains its normal meaning on the head because it still means toward the tail end of the animal. Rostral means toward the tip of the nose (rostrum). An animal’s eyes are located rostral to its ears. In humans the term nasal means toward the nose.

Dorsal and ventral refer to “up and down” directions or positions with the animal in a standing position. Dorsal means toward the back (top surface) of a standing animal, and ventral means toward the belly (bottom surface) of a standing animal. Dorsal and ventral are easiest to visualize in a standing animal, but they retain their meanings regardless of the animal’s position. When one prepares to ride a horse, the saddle is placed on the animal’s dorsal surface, and the cinch goes around the horse’s ventral surface. In humans posterior takes the place of dorsal, and anterior takes the place of ventral.

Medial and lateral refer to positions relative to the median plane. Medial means toward the median plane (toward the center line of the body), and lateral means away from the median plane. The medial surface of an animal’s leg is the one closest to its body. The lateral surface of the leg is the outer surface.

Deep (internal) and superficial (external) refer to the position of something relative to the center or surface of the body or a body part. Deep means toward the center of the body or a body part.(Internal is sometimes used in place of deep.) Superficial means toward the surface of the body or a body part. (External is sometimes used in place of superficial.) The deep digital flexor muscle is located closer to the center of the leg than the superficial digital flexor muscle, which is located nearer to the surface of the leg.

Proximal and distal are used to describe positions only on extremities, such as legs, ears, and tail, relative to the body. Proximal means toward the body, and distal means away from the body. The proximal end of the tail attaches it to the body. The toes are located on the distal end of the leg.

When it comes to describing the front and back surfaces of the legs, things get just a little more complicated. There are different terms depending on whether we are referring to the distal or proximal parts of the legs. The proximal–distal dividing line for the front leg is the proximal end of the carpus (equivalent to our wrist), and the dividing line for the rear leg is the proximal end of the tarsus (equivalent to our ankle). The back surface of the front leg from the carpus distally is called the palmar surface—like the palm of our hand—and proximal to the carpus it is the caudal surface. The back of the hind leg from the tarsus distally is called the plantar surface—like the plantar or ground surface of our foot—and proximal to the tarsus it is called the caudal surface, just like the front leg. The “front” surface of both the front and hind legs is termed dorsal from the carpus and tarsus distally and cranial proximal to them.

TABLE 1-2 Directional Terms: Domestic Animals versus Humans

Individual’s leftLeftLeft
Individual’s rightRightRight
Toward the head end of the bodyCranialSuperior
Toward the tip of the nose (head only)RostraNasal
Toward the tail end of the bodyCaudalInferior
Toward the backDorsalPosterior
Toward the bellyVentralAnterior
Toward the median planeMedialMedial
Away from the median planeLateralLateral
Toward the center (whole body or part)Deep (internal)Deep (internal)
Toward the surface (whole body or part)Superficial (external)Superficial (external)
Toward the body (extremity)ProximalProximal
Away from the body (extremity)DistalDistal
“Back” of forelimb from carpus distallyPalmarPalmar
“Back” of hindlimb from tarsus distallyPlantarPlantar
“Front” of forelimb and hindlimb from carpus and tarsus distallyDorsalAnterior

TABLE 1-3 Common Regional Terms

BarrelTrunk of the body—formed by the rib cage and the abdomen
BrisketArea at the base of the neck between the front legs that covers the cranial end of the sternum
CannonLarge metacarpal or metatarsal bone of hoofed animals
FetlockJoint between cannon bone (large metacarpal/metatarsal) and the proximal phalanx of hoofed animals
FlankLateral surface of the abdomen between the last rib and the hind legs
KneeCarpus of hoofed animals
MuzzleRostral part of the face formed mainly by the maxillary and nasal bones
PasternArea of the proximal phalanx of hoofed animals
PollTop of the head between the bases of the ears
StifleFemorotibial/femoropatellar joint— equivalent to human knee
TailheadDorsal part of the base of the tail
WithersArea dorsal to scapulas

Common Regional Terms

Common regional terms (Figure 1-3) give us a shorthand way of recording anatomic locations in veterinary records. It is easier to refer to the “fetlock” of a horse than to have to write “the joint between the large metacarpal or metatarsal bone and the proximal phalanx.” Table 1-3 gives the meanings of commonly used regional terms, including some that are unique to the horse and other hoofed animals.

FIGURE 1-3 Common regional terms.

General Plan of the Animal Body

Before studying the individual parts of the animal body, let’s take a look at the overall arrangement of the body. Our focus is on the principle of bilateral symmetry, the two main cavities (spaces) in the body, and the levels of organization that make up the body.

Bilateral Summary

Bilateral symmetry means that the left and right halves of an animal’s body are essentially mirror images of each other. Although not absolute, the principle of bilateral symmetry accurately reflects the basic inner and outer structure of the body. Paired structures, such as the kidneys, lungs, and legs, are approximately mirror images. For example, in looking at your hands, you see that they are not identical—the thumb of one of your hands is where the little finger is on the other hand—but they are mirror images of each other. Paired internal organs are similar.

Single structures in the body are generally found near the center of the body, near the median plane. This is true of structures such as the brain, the heart, and the gastrointestinal (GI) tract. At first glance the GI tract does not seem to obey this rule. After all, it is extensively folded and more or less fills the abdominal cavity. Actually the GI tract is located near the median plane, but it is so long that it has to be intricately folded so it fits in the abdomen. If we were to stretch it out, it would form one long tube. Even with all its twists, turns, and convolutions, the GI tract does not wander far from the median plane.

Body Cavities

The animal body has two main cavities (spaces)—a small dorsal cavity and a much larger ventral cavity (Figure 1-4).

FIGURE 1-4 Body cavities

Dorsal Body Cavity

The dorsal body cavity contains the brain and spinal cord, that is, the central nervous system. It consists of two parts: a somewhat spherical cranial cavity in the skull and a long, narrow spinal cavity running down the spine. The cranial cavity is also known as the cranium. It is formed from several bones of the skull, and it houses and protects the brain. The spinal cavity is also known as the spinal canal. It is formed from the vertebrae of the spine, and it houses and protects the spinal cord.

Ventral Body Cavity

The ventral body cavity is much larger than the dorsal one. It contains most of the soft organs (viscera) of the body. It is divided by the thin diaphragm muscle into the cranial thoracic cavity, also known as the thorax or chest, and the caudal abdominal cavity, also known as the abdomen.

Major structures in the thoracic cavity include the heart, lungs, esophagus, and many major blood vessels coming to and going from the heart. All of the organs in the thoracic cavity are covered by a thin membrane called the pleura. Even the cavity itself is lined by pleura. The layer that covers the organs is called the visceral layer of pleura because it lies right on the viscera (the organs). The layer that lines the whole thoracic cavity is called the parietal layer of pleura. The potential space between the two layers is filled with a small amount of lubricating fluid. The smooth pleural sur[1]faces are lubricated by the pleural fluid to ensure that the two surfaces slide over each other easily during breathing. If the pleural surfaces become thickened and roughened by inflammation, a condition called pleuritis or pleurisy, the surfaces scrape over each other with each breath, making breathing very painful.

The abdomen contains the digestive, urinary, and reproductive organs. It is lined by a thin membrane called the peritoneum, which also covers its contents. The visceral layer of peritoneum covers the abdominal organs, and the parietal layer lines the abdominal cavity. As in the thorax, a potential space filled with peritoneal fluid separates the two layers. Inflammation of the peritoneum (peritonitis) is very painful and most commonly results either from a wound that penetrates into the abdomen from the outside or from a rupture or perforation of the GI tract. When performing surgery on the digestive tract, we must take care to suture it securely closed to prevent leakage, which could lead to peritonitis.

Levels of Organization:


Cells are the basic functional units of animal life—the smallest subdivisions of the body that are capable of life. A simple, single-celled animal like an ameba has to carry out all the life functions necessary to support itself within its one cell. It must do things such as: grow; respond to positive and negative stimuli; seek out, engulf, and absorb food; eliminate wastes; and reproduce. It has no ability to influence its environment and has to take things as they come. If environmental conditions are favorable, the amoeba survives. If not, it dies.

In the complex animals we discuss in this book, the body’s cells must divide the work. The sheer size of a dog or horse results in most of the animal’s cells being far removed from the outside environment. The animal’s body must create and support an internal environment that allows all of its cells to live and function. To accomplish this, cells must specialize in some functions and eliminate others. For example, some cells specialize in absorbing nutrients (intestinal lining cells), others in carrying oxygen (red blood cells), and still others in organizing and controlling body functions (nerve cells). A particular cell in the body depends on the rest of the body’s cells all doing their jobs to ensure its survival. At the same time, all the other cells in the body rely on that cell doing its job to contribute to their survival.


When specialized cells group together, they form tissues. The entire animal body is made up of only four basic tissues: epithelial tissue, connective tissue, muscle tissue, and nervous tissue. The basic characteristics and functions of the four body tissues are summarized in Table 1-4.

Epithelial tissue is composed entirely of cells, and its main jobs are to cover body surfaces, secrete materials, and absorb materials. The surface of the skin is covered by epithelium, as are the linings of the mouth, intestine, and urinary bladder. Epithelial tissue also forms glands, which are structures that secrete useful substances and excrete wastes. The secreting units of sweat glands, salivary glands, and mammary glands are all composed of specialized epithelial tissues. The epithelium that lines the GI tract is specialized to absorb nutrients from the lumen of the tube.

Connective tissue holds the body together (connects its cells) and gives it support. Cells are very soft and cannot support themselves without outside help. Connective tissue range from very soft, such as adipose tissue (commonly called fat) to very firm, such as cartilage and bone. Connective tissues are composed of cells and a variety of nonliving intercellular substances, such as fibers, that add strength.

Muscle tissue moves the body inside and out. It exists as three types: skeletal muscle, cardiac muscle, and smooth muscle. Skeletal muscle moves the bones of the skeleton and is under conscious nervous system control. Cardiac muscle makes up the heart and works “automatically” (no conscious effort is required). Smooth muscle is found in internal organs such as the digestive tract and urinary bladder. It also works pretty much automatically.

Nervous tissue transmits information around the body and controls body functions. It transmits sensory information from the body to the brain, processes the information, and sends instructions out to tell the body how to react to changing conditions.

Epithelial and connective tissues are discussed in more detail in Chapter 5. Muscle and nervous tissues are more complicated, so each has its own chapter.

TABLE 1-4 Body Tissues

EpithelialComposed only of cellsCovers and protects (surfaces)
  Secretes (glands)
  Absorbs (intestinal lining)
ConnectiveComposed of living cells and non-living intercellular substancesBinds cells and structures together and supports the body
MuscleSkeletal (voluntary)Movements
 Cardiac (heart) 
 Smooth (involuntary) 
NervousComposed of nerve cells (neurons) and supporting cellsTransmits information around body; Coordinates and controls activities


The next level up from tissues is organs. Organs are made up of groups of tissues that work together for common purposes. For example, the kidney is an organ composed of various tissues that function together to eliminate wastes from the body. Some organs, such as the eyes, lungs, and kidneys, occur in pairs. Others, such as the brain, heart, and uterus, are single structures.


Systems are the most complex level of body organization. Systems are groups of organs that are involved in a common set of activities. For example, the digestive system is concerned with obtaining, digesting, and absorbing nutrients to fuel the rest of the body. It is composed of the organs that make up the digestive tube, such as the esophagus, stomach, and intestine, as well as accessory digestive organs, such as the salivary glands, pancreas, and liver. Table 1-1 lists all the major systems of the body.


The term health has a lot of meanings. Probably the simplest way to think of health is as a state of normal anatomy and physiology. When the structures or functions of the body become abnormal, disease results. Maintaining health is a complicated process. In terms of the levels of organization of the body, the health of the body as a whole depends on the health and proper functioning of each of its systems, organs, tissues, and cells. On the other hand, each of the body’s cells depends on the health and proper functioning of all the tissues, organs, systems, and the body as a whole. All structures and functions in the body are interrelated; nothing takes place in isolation. We can represent these interrelationships with the following diagram:


Imagine that you are driving a car. To reach your destination, you cannot just put the car in gear and then sit back, relax, and expect it automatically to take you there. You have to be actively involved in the process. You must accelerate to the proper speed, monitor and avoid other traffic, steer as the road twists and turns, accelerate up hills, brake while going down hills, stop when necessary, and generally oversee conditions and make adjustments throughout the journey. This description is an analogy for homeostasis in the body. The road is life, the car is the animal’s body, and homeostasis is all the little inputs and corrections necessary to keep the body (car) alive (on the road).

Homeostasis is the maintenance of a dynamic equilibrium in the body. The word dynamic implies activity, energy, and work, and equilibrium refers to balance. Together they summarize all the physiological processes that actively maintain balance in the various structures, functions, and properties of the body. Consider this: An animal’s body temperature cannot vary more than a few degrees from either side of the normal range without starting to interfere with other body functions. Or consider how acid–base balance, fluid balance, hormone levels, nutrient levels, and oxygen levels cannot vary by much if the body is to operate normally; they must be kept within fairly narrow operational ranges. The processes that monitor and adjust all the various essential parameters of the body are summarized by the term homeostasis.

Is some particular part of the body responsible for homeostasis? The answer is no. The whole body is responsible for homeostasis. All the body systems are involved in the many mechanisms of homeostasis, which require a lot of energy and work. Like all the little inputs and corrections that keep a car safely traveling down the road, the various homeostatic mechanisms in the body keep it functioning amid the twists and turns of life. To put it more mechanistically, the processes of homeostasis help maintain a fairly constant internal environment in the body as conditions inside and outside the animal change. Along with normal functioning of the body’s cells, tissues, organs, and systems, the processes of homeostasis make life possible.

If you are beginning to view the concepts of life and health as a little less ordinary and more unique, you are starting to appreciate the amazing complexity of the animal body. Only by understanding what is normal in the body can we hope to help sick or injured animals. With this in mind, we can proceed with our examination of the fascinating machine that is the animal body.

Clinical Application

Homeostasis and Congestive Heart Failure

The processes in the body that try to maintain the functioning of a failing heart offer some excellent illustrations of how important homeostasis is as it attempts to maintain the health and life of an animal. Congestive heart failure is a clinical term used to describe a heart that is not pumping adequate amounts of blood. This results in blood “backing up” in the body, which produces congestion, or abnormal fluid accumulation, upstream from the failing heart. There are many causes and forms of congestive heart failure, but the overall homeostatic mechanisms that attempt to maintain normal blood circulation in the body are basically the same.

The first indication that the heart is starting to fail is a drop in the cardiac output, that is, the amount of blood the heart pumps out per minute. The decreased blood flow and blood pressure are picked up by receptors in the vascular system and relayed to the central nervous system. Signals then go out to activate the sympathetic portion of the nervous system. This system, also called the fight-or-flight system, helps prepare the body for intense physical activity. Its effect on the cardiovascular system is to increase blood flow and blood pressure by stimulating the heart to beat harder and faster and by constricting blood vessels. In the short term, these mechanisms help bring blood flow and blood pressure back up to normal levels.

Unfortunately, these compensatory mechanisms cause the weak heart to work harder, which is kind of like whipping an exhausted horse to get it to move faster or pull harder. The result is a further weakening of the heart and further decreases in cardiac output. This causes more sympathetic nervous system stimulation. The cycle continues to repeat until either the heart gives up completely or we intervene with medical therapy. Homeostatic mechanisms cannot change the basic defects that are causing the heart to fail, but they help the damaged heart maintain vital blood flow to the rest of the body for as long as possible. By adding good medical care to the body’s natural homeostatic mechanisms, we can often extend the length and quality of life of an animal in congestive heart failure.

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