Descriptive Terms Useful in the Study of Anatomy
Microscopic Anatomy: Animal Cells and Tissues
Epithelial Tissues Connective Tissues Muscle Tissue Nervous Tissue
The General Plan of the Animal Body
The term anatomy has come to refer to the science that deals with the form and structure of all organisms. Literally, the word means to cut apart; it was used by early anatomists when speaking of complete dissection of a cadaver.
In contrast to anatomy, which deals primarily with structure, physiology is the study of the integrated functions of the body and the functions of all its parts (systems, organs, tissues, cells, and cell components), including biophysical and biochemical processes.
When anatomy and physiology courses are taught separately, the approach to the laboratory portion of each course is considerably different. Study in a typical gross anatomy laboratory is based primarily on dissection of animal cadavers. These usually have been preserved by embalming, and one or more parts of the vascular system have been injected with a colored material to facilitate identification of the vessels. Careful dissection coupled with close observation gives the student a concept of the shape, texture, location, and relations of structures visible to the unaided eye that can be gained in no other way. Similarly, the use of the microscope with properly prepared tissue sections on slides is essential for understanding structures that are so small they cannot be seen without optical or electron microscopic assistance.
In the physiology laboratory, the student studies the response of whole animals, isolated organs, or individual cells to changes in their environment (both internal and external).
Changes may be induced by almost any agent or manipulation, for example, drugs, changes in temperature or altitude, surgical modifications (such as neutering), and changes in diet. Monitoring of the responses may be as simple as monitoring changes in body weight or as complex as measuring the electrical potential across the cell membrane of a single cell.
Anatomists and physiologists working in research use some of the same techniques that are used in teaching laboratories but with considerable refinement. Both types of scientists use equipment and methods developed in the physical sciences, particularly chemistry and physics. The anatomist applies the principles of physics to the use of microscopes and applies knowledge of chemistry in the staining of various parts of cells and tissues. The combination of chemistry and microscopic anatomy is known as histochemistry.
Although anatomy and physiology are commonly pursued as more or less independent disciplines, they are both facets of the study of the animal body. A thorough knowledge of structure imparts much information about its function. However, a mere description of structure without describing function would be of little practical value. Conversely, it is impossible to gain a thorough understanding of function without a basic knowledge of structure.
The science of anatomy has become so extensive that it is now divided into many specialized branches. In fact, Dorland's Medical Dictionary defines 30 subdivisions of anatomy. This text chiefly describes gross (macroscopic) anatomy. This is the study of the form and relations (relative positions) of the structures of the body that can be seen with the unaided eye. Comparative anatomy is a study of the structures of various species of animals, with particular emphasis on those characteristics that aid in classification. Embryology is the study of developmental anatomy, covering the period from conception (fertilization of the egg) to birth. Another large branch of anatomy consists of the study of tissues and cells that can be seen only with the aid of a microscope. This is known as microscopic anatomy, or histology.
The most recent development in the study of anatomy is ultrastructural cytology, which deals with portions of cells and tissues as they are visualized with the aid of the electron microscope. The term fine structure is used frequently in reference to structures seen in electron micrographs (photographs made with the electron microscope).
Our approach to the study of anatomy will be chiefly by systems-systematic anatomy. To name the study, the suffix -ology, which means branch of knowledge or science, is added to the root word referring to the system. Table 1-1 indicates the commonly accepted systems, the name of the study of those systems, and the chief structures involved in each system.
Physiology has also become so extensive in scope that many areas of specialization are recognized. Like anatomy, these may be based on body systems (e.g., neurophysiology, gastrointestinal physiology, cardiovascular physiology, respiratory physiology, endocrine physiology, and reproductive physiology) or the level of biological organization (cell physiology and organismal physiology). All of these subdivisions become the parts of such overall areas of study as applied physiology, comparative physiology, pathophysiology, medical physiology, and mammalian physiology. We will be concerned with these systems and studies as they relate specifically to farm animals.
Descriptive Terms Useful in the Study of Anatomy
When giving geographic locations, we make use of certain arbitrary frames of reference known as meridians of latitude and longitude. However, since an animal is rarely oriented exactly with a line on the earth's surface, our frames of reference must be in relation to the animal itself and must apply regardless of the position or direction of the animal (Fig. 1-1). Many terms of direction differ significantly between human and domestic animal anatomy because of the orientation of bipedal versus quadrupedal stance. Although use of human anatomical nomenclature in quadrupeds usually leads to confusion, the terms anterior, posterior, superior, and inferior are frequently used to describe the eye and aspects of dental anatomy of both human beings and domestic animals (see Chapters 11 and 12).
Cranial is a directional term meaning toward the head. The shoulder is cranial to the hip; it is closer to the head than is the hip.
Caudal means toward the tail. The rump is caudal to the loin.
Rostral and caudal are directional terms used in reference to features of the head to mean toward the nose (rostral) or toward the tail (caudal).
The median plane is an imaginary plane passing through the body so as to divide the body into equal right and left halves. A beef carcass is split into two halves on the median plane.
A sagittal plane is any plane parallel to the median plane. The median plane is sometimes called the midsagittal plane.
A transverse plane is at right angles to the median plane and divides the body into cranial and caudal segments. A cross-section of the body would be made on a transverse plane. The cinch of a saddle defines a transverse plane through the thorax of a horse.
A horizontal plane is at right angles to both the median plane and transverse planes. The horizontal plane divides the body into dorsal (upper) and ventral (lower) segments. If a cow walks into a lake until the water comes above the chest, the surface of the water is in a horizontal plane in relation to the cow.
In addition to the planes of reference, other descriptive terms are valuable in defining an area we wish to discuss.
Medial is an adjective meaning close to or toward the median plane. The heart is medial to the lungs; it is closer to the median plane than are the lungs. The chestnut is on the medial aspect (inside) of a horse's limb; it is on the side closest to the median plane.
Lateral is the antonym of medial; it means away from the median plane. The ribs are lateral to the lungs, that is, farther from the median plane.
Dorsal means toward or beyond the backbone or vertebral column. The kidneys are dorsal to the intestines; they are closer to the vertebral column. Dorsum is the noun referring to the dorsal portion or back. A saddle is placed on the dorsum of a horse.
Ventral means away from the vertebral column or toward the midabdominal wall. The udder is the most ventral part of the body of a cow, the part of the body farthest from the vertebral column.
Deep and internal indicate proximity to the center of an anatomical structure. The humerus (arm bone) is deep in relation to all other structures in the arm.
Superficial and external refer to proximity to the surface of the body. Hair is superficial to all other structures of the body.
Proximal means relatively close to a given part, usually the vertebral column, body, or center of gravity. Proximal is generally used in reference to an extremity or limb. The carpus or knee is proximal to the foot.
Distal means farther from the vertebral column, and like proximal, it is generally used in reference to portions of an extremity. The hoof is distal to the carpus or knee.
The suffix -ad is used to form an adverb from any of the above-named directional terms, indicating movement in the direction of or toward, as in dorsad, ventrad, caudad, and craniad, that is, respectively, toward the dorsum, toward the belly, toward the tail, and toward the head. For example, the superficial digital flexor tendon inserts on the distal limb (the adjective distal describes noun limb), but it passes distad as it runs along the palmar aspect of the manus (the adverb distad describes the verb passes).
In describing the thoracic limb (forelimb) distal to (below) the carpus, palmar refers to the flexor or caudal surface. Dorsal is used in this region to refer to the opposite (cranial) side. In describing the pelvic limb (hindlimb) distal to the hock, plantar refers to the caudal surface, and dorsal here, too, refers to the side directly opposite (the cranial side).
Prone refers to a position in which the dorsal aspect of the body or any extremity is uppermost. Pronation refers to the act of turning toward a prone position.
Supine refers to the position in which the ventral aspect of the body or palmar or plantar aspect of an extremity is uppermost. Supination refers to the act of turning toward a supine position.
The term median is often confused with medial. Both words are used as adjectives when describing anatomical structures. Median means on the midline (as in the median plane, or the median artery). Medial is subtly different, as it means toward the midline and is a term of relativity (as it implies that there is a lateral).
Microscopic Anatomy: Animal Cells and Tissues
All living things, both plants and animals, are constructed of small units called cells. The simplest animals, such as the ameba, consist of a single cell that is capable of performing all functions commonly associated with life. These functions include growth (increase in size), metabolism (use of food), response to stimuli (such as moving toward light), contraction (shortening in one direction), and reproduction (development of new individuals of the same species).
A typical cell consists of three main parts, the cytoplasm, the nucleus, and the cell membrane (Fig. 1-2). Detailed structure of the individual cell is described in Chapter 2. Tissues are discussed in this chapter.
In complex animals, certain cells specialize in one or more the functions of the animal body. A group of specialized cells is a tissue. For example, cells that specialize in conducting impulses make up nerve tissue. Cells that specialize in holding structures together make up connective tissue. Various tissues are associated in functional groups called organs. The stomach is an organ that functions in digestion of food. A group of organs that participate in a common enterprise make up a system. The stomach, liver, pancreas, and intestines are all part of the digestive system.
The primary types of tissues include (1) epithelial tissues, which cover the surface of the body, line body cavities, and form glands; (2) connective tissues, which support and bind other tissues together and from which, in the case of bone marrow, the formed elements of the blood are derived; (3) muscle tissues, which specialize in contracting; and (4) nervous tissues, which conduct impulses from one part of the body to another.
Epithelial Tissues
In general the epithelial tissues are classified as simple (composed of a single layer) or stratified (many-layered). Each of these types is further subdivided according to the shape of the individual cells within it (Fig. 1-3). Simple epithelium includes squamous (platelike) cells, cuboidal (cubic) cells, columnar (cylindrical) cells, and pseudostratified columnar cells.
Simple squamous epithelium consists of thin, platelike cells. They are much expanded in two directions but have little thickness. The edges are joined somewhat like mosaic tile covering a floor. A layer of simple squamous epithelium has little tensile strength and is found only as a covering layer for stronger tissues. Simple squamous epithelium is found where a smooth surface is required to reduce friction. The coverings of viscera and the linings of body cavities and blood vessels are all composed of simple squamous epithelium.
Cuboidal epithelial cells are approximately equal in all dimensions. They are found in some ducts and in passageways in the kidneys. The active tissue of many glands is composed of cuboidal cells.
Columnar epithelial cells are cylindrical. They are arranged somewhat like the cells in a honeycomb. Some columnar cells have whiplike projections called cilia extending from the free extremity.
Pseudostratified columnar epithelium is composed of columnar cells. However, they vary in length, giving the appearance of more than one layer or stratum. This type of epithelium is found in the upper respiratory tract, where the lining cells are ciliated. Stratified epithelium consists of more than one layer of epithelial cells and includes stratified squamous, stratified columnar, and transitional epithelia.
Stratified squamous epithelium forms the outer layer of the skin and the lining of the first part of the digestive tract as far as the stomach. In ruminants, stratified squamous epithelium also lines the forestomach (rumen, reticulum, and omasum). Stratified squamous epithelium is the thickest and toughest of the epithelia, consisting of many layers of cells. From deep to superficial, these layers include the basal layer (stratum basale), the parabasal layer (stratum spinosum), intermediate layer (stratum granulosum), and superficial layer (stratum corneum). The deepest layer, the stratum basale, contains the actively growing and multiplying cells. These cells are somewhat cuboidal, but as they are pushed toward the surface, away from the blood supply of the underlying tissues, they become flattened, tough, and lifeless and are constantly in the process of peeling off. This layer of cornified (keratinized) dead cells becomes very thick in areas subjected to friction. Calluses are formed in this manner.
Stratified columnar epithelium is composed of more than one layer of columnar cells and is found lining part of the pharynx and salivary ducts.
Transitional epithelium lines the portions of the urinary system that are subjected to stretching. These areas include the urinary bladder and ureters. Transitional epithelium can pile up many cells thick when the bladder is small and empty and stretch out to a single layer when completely filled.
Glandular epithelial cells are specialized for secretion or excretion. Secretion is the release from the gland cell of a substance that has been synthesized by the cell and that usually affects other cells in other parts of the body. Excretion is the expulsion of waste products.
Glands may be classified either as endocrine glands (glands without ducts, which empty their secretory products directly into the bloodstream), or as exocrine glands (glands that empty their secretory products on an epithelial surface, usually by means of ducts).
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Excerpted from Anatomy and Physiology of Farm Animals by Rowen D. Frandson W. Lee Wilke Anna Dee Fails Copyright © 2009 by Rowen D. Frandson, W. Lee Wilke, Anna Dee Fails. Excerpted by permission.
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