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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
The Seventh Edition of Anatomy and Physiology of Farm Animals is a thoroughly updated and revised version of this classic text. Drawing on current science and terminology with a number of new illustrations throughout and a new chapter on poultry, the book maintains its reputation for clarity, balanced scope, and breadth of content. The Seventh Edition provides veterinary, animal science, agriculture, and veterinary technician students with a comprehensive yet clear reference to understanding the fundamentals of anatomy and physiology.
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Seitenzahl: 964
Veröffentlichungsjahr: 2013
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TABLE OF CONTENTS
PREFACE
ACKNOWLEDGMENTS
Chapter 1 INTRODUCTION TO ANATOMY AND PHYSIOLOGY
Descriptive Terms Useful in the Study of Anatomy
Microscopic Anatomy: Animal Cells and Tissues
The General Plan of the Animal Body
Chapter 2 ANATOMY AND PHYSIOLOGY OF THE CELL
Properties of Life
Chemical Composition of the Cell
Microscopic Study of the Cell
The Cell Membrane
Transport Across Cell Membranes
Membrane Potentials and Excitable Cells
Membrane Receptors and Intracellular Signaling
Cytoplasm and Cytoplasmic Organelles
Nucleus
Cell Division
Regulation of Cell Growth and Replication
Chapter 3 EMBRYOLOGY
Development of Germ Layers
Neurulation
Differentiation of Other Tissues
Chapter 4 THE SKELETAL SYSTEM
Functions of Bones
Terminology
Classification of Bones According to Gross Appearance
Axial Skeleton
Appendicular Skeleton
Chapter 5 MICROSCOPIC ANATOMY AND GROWTH AND DEVELOPMENT OF BONE
Microscopic Anatomy and Formation of Bone
Ossification
Physiology of Bone
Fractures and Fracture Healing
Other Pathologic Conditions
Chapter 6 JOINTS
Classification of Joints
Movements of Joints
Types of Synovial Joints
Joints of the Axial Skeleton
Joints of the Appendicular Skeleton
Pathology of Joints and Related Structures
Chapter 7 Anatomy of the Muscular System
Types of Muscle Tissue
Functions of the Muscular System
Skeletal Muscle Organization
Muscles of the Thoracic Limb
Muscles of the Pelvic Limb
Muscles of the Head
Muscles of the Trunk and Neck
Chapter 8 MICROSCOPIC ANATOMY AND PHYSIOLOGY OF MUSCLE
Skeletal Muscle
Smooth Muscle
Cardiac Muscle
Chapter 9 ANATOMY OF THE NERVOUS SYSTEM
Microscopic Neuroanatomy
Embryology
Central Nervous System
Peripheral Nervous System
Autonomic Nervous System
Enteric Nervous System
Chapter 10 PHYSIOLOGY OF THE NERVOUS SYSTEM
Physiology of the Nerve Impulse
Synaptic Transmission
Neurotransmitters
Neural Control of Skeletal Muscle
Physiology of the Autonomic Nervous System
Regeneration and Repair in the Nervous System
Chapter 11 SENSE ORGANS
Sensory Receptors
Somatosensation
Visceral Sensations
Chemical Senses
Hearing and Balance
Vision
Chapter 12 ENDOCRINOLOGY
Hormones and Their Receptors
Cellular Effects of Peptide Hormones
Cellular Effects of Steroid and Thyroid Hormones
Negative and Positive Feedback Regulation
Hypothalamopituitary Axis
Hormones of the Neurohypophysis
Hormones of the Adenohypophysis
Other Endocrine Glands
Chapter 13 THE INTEGUMENT
Integument
Skin
Adnexa of the Skin
Modified Epidermis
Coat Color in Horses
Wool
Chapter 14 THE EQUINE FOOT AND PASSIVE STAY APPARATUS
Structure of the Foot
Function
Stay Apparatus
Chapter 15 BLOOD AND OTHER BODY FLUIDS
Blood
Plasma and Serum
Blood pH
Hemostasis and Coagulation
Lymph
Serous Fluids
Chapter 16 BODY DEFENSES AND THE IMMUNE SYSTEM
Nonspecific Defenses
Specific Immune Response
B Lymphocytes
Immunoglobulins
T Cells and Cell-Mediated Immunity
Lymphocyte Origin, Development, and Residence
Active and Passive Immunities
Immunological Surveillance
Lymphatic System
Chsapter 17 ANATOMY OF THE CARDIOVASCULAR SYSTEM
Heart
Vessels
Pulmonary Circulation
Systemic Circulation
Veins
Fetal Circulation
Chapter 18 PHYSIOLOGY OF THE HEART AND CIRCULATION
Basic Design and Function of the Cardiovascular System
Cardiac Cycle
Electrical Activity of the Heart
Cardiac Output and Its Regulation
Structure and Function of Blood Vessels
Regulation of Arterial Blood Pressure and Blood Volume
Cardiovascular Function During Exercise and Hypovolemia
Chapter 19 THE RESPIRATORY SYSTEM
Upper Respiratory Tract
Thorax
Physiology of Respiration
Chapter 20 ANATOMY OF THE DIGESTIVE SYSTEM
Organization of the Digestive System
Mouth
Pharynx
Esophagus
Nonruminant Stomach
Ruminant Stomach
Small Intestine
Large Intestine
Peritoneal Features
Accessory Digestive Organs
Chapter 21 PHYSIOLOGY OF DIGESTION
Pregastric Physiology
Ruminant Forestomach
Gastric Physiology
Physiology of the Small Intestine, Exocrine Pancreas, and Liver
Physiology of the Cecum and Colon
Rectum and Defecation
Chapter 22 NUTRITION AND METABOLISM
Nutrition
Metabolism
Chapter 23 THE URINARY SYSTEM
Anatomy of the Kidney
Ureters, Urinary Bladder, and Urethra
Micturition
Overview of Function and Histology of the Kidneys
Glomerular Filtration
Proximal Tubule Transport
Concentration and Dilution of Urine: Role of the Loop of Henle and Collecting Duct Transport
Sodium, Potassium, and Aldosterone
Urine Acidification
Regulation of Acid-Base Balance
Chapter 24 ANATOMY OF THE MALE REPRODUCTIVE SYSTEM
Testis
Epididymis
Ductus Deferens
Scrotum
Inguinal Canal
Descent of the Testis
Castration
Accessory Sex Glands
Penis
Prepuce
Muscles of the Male Genitalia
Blood and Nerve Supply of the Male Genitalia
Chapter 25 PHYSIOLOGY OF MALE REPRODUCTION
Seminiferous Tubules and Spermatogenesis
Epididymis
Semen and Semen Technology
Hormones of Male Reproduction
Erection and Ejaculation
Chapter 26 ANATOMY OF THE FEMALE REPRODUCTIVE SYSTEM
Ovaries
Uterine Tubes
Uterus
Vagina
Vestibule and Vulva
Blood and Nerve Supply of the Female Reproductive Tract
Chapter 27 THE OVARY AND ESTROUS CYCLES
Oogenesis
Ovulation
Corpus Luteum
Phases of the Estrous Cycle
Specifics of Selected Estrous Cycles
Chapter 28 PREGNANCY AND PARTURITION
Fertilization
Implantation and Placentation
Hormones of Pregnancy
Pregnancy Diagnosis
Parturition
Fetal Presentations and Delivery
Dystocia
Chapter 29 ANATOMY AND PHYSIOLOGY OF THE MAMMARY GLANDS
Mammary Glands of the Cow
Microscopic Anatomy of the Mammary Gland
Mammary Glands of Swine
Mammary Glands of Sheep and Goats
Mammary Glands of the Horse
Physiology of Lactation
Chapter 30 POULTRY
Integument
Body Design
Skeleton and Bone
Musculature
Gastrointestinal System
Respiratory System
Cardiovascular System
Lymphatic System
Urinary System
Female Reproductive System
Male Reproductive System
Sex Chromosomes
Reproduction and Photoperiods
APPENDIX: ABBREVIATIONS
BIBLIOGRAPHY
INDEX
PLATE 1: TISSUE TYPES
PLATE 2: TYPES OF BLOOD CELLS
Seventh Edition first published 2009© 2009 Wiley-Blackwell
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First Edition, 1965, Lippincott Williams & WilkinsSecond Edition, 1974, Lippincott Williams & WilkinsThird Edition, 1981, Lippincott Williams & WilkinsFourth Edition, 1986, Lippincott Williams & WilkinsFifth Edition, 1992, Lippincott Williams & WilkinsSixth Edition, 2003, Lippincott Williams & Wilkins
Library of Congress Cataloging-in-Publication Data
Wilke, W. Lee.
Anatomy and physiology of farm animals / W. Lee Wilke, Anna Dee Fails, R.D. Frandson. – 7th ed.
p. ; cm.
R.D. Frandson’s name appears first on the 6th ed.Includes bibliographical references and index.ISBN-13: 978-0-8138-1394-3 (alk. paper)ISBN-10: 0-8138-1394-8 (alk. paper)
1. Veterinary anatomy. 2. Veterinary physiology. I. Fails, Anna Dee. II. Frandson, R. D. III. Title.[DNLM: 1. Anatomy, Veterinary. 2. Animals, Domestic–physiology. SF 761 W681a 2009]SF761.F8 2009636.089′2–dc22
2008051665
A catalog record for this book is available from the U.S. Library of Congress.
Disclaimer
The publisher and the author make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation warranties of fitness for a particular purpose. No warranty may be created or extended by sales or promotional materials. The advice and strategies contained herein may not be suitable for every situation. This work is sold with the understanding that the publisher is not engaged in rendering legal, accounting, or other professional services. If professional assistance is required, the services of a competent professional person should be sought. Neither the publisher nor the author shall be liable for damages arising herefrom. The fact that an organization or Website is referred to in this work as a citation and/or a potential source of further information does not mean that the author or the publisher endorses the information the organization or Website may provide or recommendations it may make. Further, readers should be aware that Internet Websites listed in this work may have changed or disappeared between when this work was written and when it is read.
1 2009
To Zelda, Eric, Stephanie and Mark
—R.D.F.
To M.W. Poore
—A.D.F.
To Aline and Willard
—W. L.W.
PREFACE
The first edition of Anatomy and Physiology of Farm Animals combined accuracy with simplicity and clarity of expression and gained considerable acceptance among veterinary and veterinary technician students as well as those majoring in the animal sciences and vocational agriculture. Later editions were revised to increase the value of the book to first-year veterinary students. Now in its seventh edition, this text maintains a strong reputation by achieving a balance in both depth and scope of its subject.
Summary of Key Features
This edition includes a number of new or updated features that further enhance the appeal of the text.
Species Orientation. As in the first six editions, general principles of anatomy and physiology are discussed as they apply to farm animals. Important species differences are described, with the most attention given to the horse and the cow. The sheep, goat, and hog are described where important and relevant species differences exist. When the goat is not mentioned specifically, it may be assumed that the goat is similar to the sheep.
Poultry Chapter. This edition includes a new chapter on anatomy and physiology of poultry. The chapter discusses all body systems, with an emphasis on anatomical features and physiological processes unique to birds. This addition further expands the scope of the text and increases its usefulness to students in a variety of diverse programs of study.
New Art. The art program in the text continues to be revised and enlarged. Approximately 60 new line drawings have been added, and many of these are original to this edition. The use of radiographic images to illustrate anatomical features is also a new addition to this edition.
Cellular and Molecular Mechanisms. In keeping with current trends in physiology and medicine, cellular and molecular mechanisms in physiological processes are emphasized, but attempts are made to illustrate the relationships between these mechanisms and phenomena that can be observed in intact animals. Where controversial subjects are discussed, the generally accepted view is given in greatest detail.
Clinical Extracts. Clinical extracts, material especially useful in a clinical setting, are highlighted in blue throughout the text. These extracts help students understand the practical value of anatomy and physiology and serve to illustrate mechanistic links between these basic sciences and clinical conditions.
Nomina Anatomica Veterinaria. Every effort has been made to bring the anatomical nomenclature used in this text into accordance with the fifth edition of the Nomina Anatomica Veterinaria. Exceptions are made only when a different term is in such common usage as to argue for an alternative name.
Glossary and Terminology. Because abbreviations may be confusing and difficult to remember, a glossary of commonly used abbreviations is included in the appendix. Technical terms are used throughout the book, but most terms not found in an ordnary college dictionary are defined within the text.
R.D. FrandsonFort Collins, Colorado
ACKNOWLEDGMENTS
Acknowledgment of all sources of information and assistance in the evolution of this book from its first edition in 1964 to this, the seventh edition, is impossible. However, I would like to thank specifically the following colleagues and friends for their many and varied contributions.
Dr. Y.Z. Abdelbaki, Dr. T.H. Belling, Jr., Miss Elsie Bergland, Dr. R.W. Davis, Dr. G.P. Epling, Dr. R.A. Kainer, and Dr. H. Meyer.
The many publishers who loaned illustrations and tables.
Dr. Charles W. Miller for the section in the fourth edition on the use of ultrasound.
Dr. Sandra Pitcaithley for assistance with microscopic images for this and the previous (sixth) edition.
Dr. Anna Fails and Dr. Lee Wilke for rewriting the fifth edition as coauthors of the sixth and seventh editions, and Dr. Fails for her original artwork for the sixth and seventh editions.
Chapter 1
INTRODUCTION TO ANATOMY AND PHYSIOLOGY
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.
Table 1-1. Nomenclature for Systematic Anatomy
System
Name of Study
Chief Structures
Skeletal system
Osteology
Bones
Articular system
Arthrology
Joints
Muscular system
Myology
Muscles
Digestive system
Splanchnology
Stomach and intestines
Respiratory system
Splanchnology
Lungs and airways
Urinary system
Splanchnology
Kidneys and urinary bladder
Reproductive system
Splanchnology
Ovaries and testes
Endocrine system
Endocrinology
Ductless glands
Nervous system
Neurology
Brain, spinal cord, and nerves
Circulatory system
Cardiology
Heart and vessels
Sensory system
Esthesiology
Eye and ear
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).
Figure 1-1. Directional terms and planes of the animal body.
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.
Figure 1-2. A cell as seen with a light microscope.
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.
Figure 1-3. Primary types of epithelial tissues. A) Simple squamous. B) Simple squamous in tubular arrangement. C) Simple cuboidal. D) Simple cuboidal arranged as a duct. E) Simple columnar. F) Pseudostratified columnar with cilia. G) Transitional. H) Stratified squamous.
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).
The endocrine glands are an important part of the control mechanisms of the body, because they produce special chemicals known as hormones. The endocrine glands are discussed in Chapter 12. Hormones carried to all parts of the body by the blood constitute the humoral control of the body. Humoral control and nervous control are the two mechanisms maintaining homeokinesis, also called homeostasis, a relatively stable but constantly changing state of the body. Humoral responses to stimuli from the environment (both external and internal) are slower and longer acting than responses generated by way of the nervous system. The nervous system is described in some detail in Chapters 9 and 10.
Collectively, the endocrine glands constitute the endocrine system, which is studied in endocrinology. However, exocrine glands are scattered throughout many systems and are discussed along with the systems to which they belong, such as the digestive, urogenital, and respiratory systems.
According to their morphologic classification (Fig. 1-4), a gland is simple if the duct does not branch and compound if it does. If the secretory portion forms a tubelike structure, it is called tubular; if the secretory portion resembles a grape or hollow ball, it is called alveolar or acinar (the terms are used interchangeably). A combination of tubular and alveolar secretory structures produces a tubuloalveolar gland.
Figure 1-4. Types of exocrine glands and comparison of simple and compound glands. A) Simple tubular gland. B) Simple coiled tubular gland. C) Simple branched tubular gland. D & E) Simple acinar/alveolar glands and simple branched acinar/alveolar glands. F) Compound tubular gland. G & H) Compound acinar/alveolar glands. Compound tubuloacinar/tubuloalveolar glands consist of either a mixture of tubular and acinar/alveolar secretory units or tubular secretory units “capped” by acini or alveoli. (Reprinted with permission of Wiley-Blackwell from Eurell, J.A. and Frappier, B.L. Dellmann’s Textbook of Veterinary Histology. 6th ed. Ames, IA: Blackwell Publishing Professional, 2006.)
Compound glands often are subdivided grossly into lobes, which in turn may be further subdivided into lobules. Hence, the connective tissue partitions (called septa) are classified as interlobar septa if they separate lobes and as interlobular septa if they separate lobules. Similar terminology may be applied to ducts draining lobes or lobules of glands, that is, interlobar ducts and interlobular ducts, respectively.
Another classification of glands is based on the manner in which their cells elaborate their secretion. By this classification, the most common type is the merocrine gland. Merocrine glands pass their secretory products through the cell wall without any appreciable loss of cytoplasm or noticeable damage to the cell membrane. The holocrine gland is the least common type. After the cell fills with secretory material, the entire holocrine gland cell discharges to the lumen of the gland to constitute the secretion. Sebaceous glands associated with hair follicles of the skin are the most common holocrine glands. An intermediate form of secretion in which a small amount of cytoplasm and cell membrane is lost with the secretion is sometimes described for the prostate and some sweat glands. Such glands are called apocrine glands.
Connective Tissues
Connective tissues, as the name implies, serve to connect other tissues. They give form and strength to many organs and often provide protection and leverage. Connective tissues include elastic tissue, collagenous (white fibrous) tissue, reticular (netlike) tissue, adipose (fat) tissue, cartilage, and bone.
Elastic tissue contains kinked fibers that tend to regain their original shape after being stretched. This tissue is found in the ligamentum nuchae, a strong band that helps to support the head, particularly in horses and cattle. Elastic tissue also is found in the abdominal tunic, in the ligamenta flava of the spinal canal, in elastic arteries, and mixed with other tissues wherever elasticity is needed.
Collagenous (white fibrous) tissue is found throughout the body in various forms. Individual cells (fibroblasts) produce long proteinaceous fibers of collagen, which have remarkable tensile strength. These fibers may be arranged in regular repeating units, or laid down in a more random, irregular arrangement.
In dense regular connective tissue (Fig. 1-5), the fibers are arranged in parallel bundles, forming cords or bands of considerable strength. These are the tendons, which connect muscles to bones, and the ligaments, which connect bones to bones.
Figure 1-5. Longitudinal section through a tendon showing the histological appearance of dense regular connective tissue. (Left) notice the line of nuclei (arrow), indicating the loose connective tissue surrounding blood vessels and nerves. Hematoxylin and eosin stain, ×226. At higher power (right), spindle-shaped fibroblasts can be seen among collagen fibers. Hematoxylin and eosin stain, ×660. (Reprinted with permission of Wiley-Blackwell from Dellmann, H.D. and Brown, E.M. Textbook of Veterinary Histology. 2nd ed. Philadelphia: Lea & Febiger, 1981.)
The fibers of dense irregular connective tissue are arranged in a thick mat, with fibers running in all directions. The dermis of the skin, which may be tanned to make leather, consists of dense irregular connective tissue. This forms a strong covering that resists tearing and yet is flexible enough to move with the surface of the body.
Areolar (loose) connective tissue (Plate I) is found throughout the body wherever protective cushioning and flexibility are needed. For example, blood vessels are surrounded by a sheath of areolar connective tissue, which permits the vessels to move and yet protects them.
Beneath the dermis is a layer of loosely arranged areolar connective tissue fibers that attaches the skin to underlying muscles. This attachment is flexible enough to permit movement of the skin. It also permits the formation of a thick layer of fat between the skin and underlying muscles. Whenever the skin is adherent to bony prominences because of a lack of areolar tissue, the skin will not move, and no layer of fat can form. This feature is seen in beef cattle that have ties; in this case, the skin over the back shows large dimples where fat cannot fill in because the skin is adherent to the vertebrae.
Reticular connective tissue consists of fine fibrils and cells. Reticular tissue makes up part of the framework of endocrine and lymphatic organs.
Adipose tissue (fat) forms when connective tissue cells called adipocytes store fat as inclusions within the cytoplasm of the cell. As more fat is stored, the cell eventually becomes so filled with fat that the nucleus is pushed to one side of the cell, which, as a result, becomes spherical (Plate I). Most fat in the animal body is white, although it may have a yellow tinge in horses and some breeds of dairy cattle because of carotenoids in the feed.
In contrast to this white fat, a small amount of brown fat may be found in domestic mammals, hibernating mammals, rodents, and human infants. The brown fat is found between the scapulae, in the axillae, in the mediastinum, and in association with mesenteries in the abdomen. Brown fat apparently generates heat to protect young mammals and hibernating mammals from extreme cold.
Cartilage is a special type of connective tissue that is firmer than fibrous tissue but not as hard as bone. The nature of cartilage is due to the structure of the intercellular material found between the chondrocytes (cartilage cells). The three types of cartilage described are hyaline, elastic, and fibrous.
Hyaline cartilage is the glasslike covering of bones within joints. This type of cartilage forms a smooth surface that reduces friction, so that one bone easily glides over another. The actively growing areas near the ends of long bones also consist of hyaline cartilage. Elastic cartilage consists of a mixture of cartilage substance and elastic fibers. This type of cartilage gives shape and rigidity to the external ear. Fibrocartilage consists of a mixture of cartilage and collagenous fibers, which forms a semielastic cushion of great strength. The intervertebral disks between the bodies of adjacent vertebrae are composed of fibrocartilage.
Bone is produced by bone-forming cells called osteoblasts. These cells produce osteoid tissue, which later becomes calcified to form bone. The bone may be arranged in the form of spicules (small spikes) and flat plates, forming a spongelike network called cancellous bone or spongy bone. Alternatively, it may be laid down in the form of laminated cylinders (Haversian or osteonal systems), closely packed together to form compact bone (Plate I).
Blood. Blood consists of a fluid matrix (liquid portion), the plasma, a variety of cells (Plate II), proteins, monosaccharides (simple sugars), products of fat degradation, and other circulating nutrients, wastes, electrolytes, and chemical intermediates of cellular metabolism. It is sometimes considered to be a connective tissue because of the origin of some of its components.
Red blood cells (RBCs) are also called erythrocytes. In most domestic mammals they are nonnucleated biconcave disks that contain the protein hemoglobin. The main function of the RBCs is to carry hemoglobin. Hemoglobin in turn has the primary function of carrying oxygen from the lungs to all tissues of the animal. At the tissue level, oxygen is released to the cells, while carbon dioxide, which is produced by the cells, diffuses into the blood to be carried back to the lungs, where it can be eliminated during breathing. Anemia is a reduction in the concentration of functional RBCs in the blood. It can result from a loss of red cells (as in hemorrhage), insufficient RBC production, or inappropriate or premature degradation of the red cells.
White cells (also called leukocytes) are one of the body’s first lines of defense against infection. They include agranulocytes and granulocytes. Agranulocytes are of two kinds: monocytes, large cells that engulf and destroy foreign particles, and lymphocytes, which usually are smaller and are associated with immune responses. An excess of agranulocytes tends to be associated with chronic types of diseases.
Granulocytes (polymorphonuclear leukocytes) are of three types and are described according to their affinity for different stains. Granules in neutrophils stain indifferently; basophils have dark-staining granules when stained with common blood stains; and eosinophils have red-staining granules. Blood platelets (thrombocytes) are small, irregularly shaped cellular fragments that are associated with the clotting of the blood. Mammalian platelets lack a nucleus.
Plasma is the fluid part of unclotted blood. Plasma is particularly useful as a substitute for blood in transfusions because the proteins in it give it the same osmotic pressure as blood. Plasma therefore will not escape from blood vessels as readily as a salt solution.
Serum is the supernatant fluid that remains after a clot forms and incorporates the cellular components of blood. It is similar to plasma but lacks most of the clotting factors. Serum is sometimes administered for prevention and treatment of diseases because it contains the antibody fractions of the blood.
Muscle Tissue
The three types of muscle tissue are skeletal, smooth, and cardiac (Plate I; Fig. 1-6). Both skeletal and cardiac muscle cells consist of fibers that under the microscope show characteristic cross-striations, so both are classified as striated muscle. Smooth muscle cells lack distinct cross-striations.
Figure 1-6. Types of muscle tissue. A) Smooth muscle. B) Skeletal muscle. C) Cardiac muscle. (Courtesy of Sandra Pitcaithley, DVM.)
Each skeletal muscle cell must have its own nerve supply, and when stimulated, the whole fiber contracts. This is the all-or-none law of muscle contraction. However, the force of contraction depends on the state of the fiber at any one moment. For example, is it already fatigued? Is it warmed up? Is it stretched? Striated skeletal muscle tissue plus some connective tissue makes up the flesh of meat-producing animals.
Smooth muscle cells are spindle-shaped cells that contain one centrally located nucleus per cell. Smooth muscle is found in the walls of the digestive tract, in the walls of blood vessels, and in the walls of urinary and reproductive organs. These cells contract more slowly than skeletal muscle and in response to a variety of stimuli, although they are not under voluntary control.
Cardiac muscle is also known as involuntary striated muscle because it is not usually under conscious control, yet it does have crossstriations. The heart muscle is composed of a complex branched arrangement of cardiac muscle cells. Modified muscle cells called Purkinje fibers conduct impulses within the heart, much as nerve fibers do in other parts of the body.
Nervous Tissue
The essential cell of nervous tissue is the neuron (nerve cell). The neuron consists of a nerve cell body and two or more nerve processes (nerve fibers). The processes are called axons if they conduct impulses away from the cell body and dendrites if they conduct impulses toward the cell body (Fig. 1-7).
Bundles of axons in the spinal cord are called tracts, and those in the periphery are called nerves. A nerve fiber may be covered by a myelin sheath, a specialized wrapping created by supportive cells called Schwann cells in nerves or by oligodendrocytes within the brain and spinal cord.
Figure 1-7. A typical motor neuron.
The special connective tissues of nervous tissue are called neuroglia and are found only in the central nervous system. Outside the central nervous system, in addition to the Schwann cells, ordinary white fibrous tissue serves as the major protective covering for the nerves.
The General Plan of the Animal Body
All farm animals are vertebrates, and as such they have a vertebral column. The body (with the exception of some of the internal organs) exhibits bilateral symmetry. This means that the right and left sides of the body are mirror images of each other. Similar right and left structures are called paired structures, such as a pair of gloves that are similar but not interchangeable. Most unpaired structures are on or near the median plane, and of course, only one of each unpaired structure exists in any given animal. The tongue, trachea, vertebral column, and heart are examples of unpaired structures. The ribs, limbs, eyes, and most muscles are paired structures.
Wherever organs are expected to be in more-or-less constant motion and must glide past one another without friction (e.g., the beating heart and moving gut), a serosal cavity is present. The simple squamous epithelium lining various body cavities is also called mesothelium, and the cavities have within them only a scant amount of fluid to facilitate free movement of the tissues. The diaphragm divides the embryonic body cavity into a thoracic cavity and the abdominopelvic cavity. Each of these are further subdivided.
The thoracic cavity contains the pericardial sac, which surrounds the heart, and two pleural sacs, which surround the two lungs. These sacs are formed by a serous membrane, the pleura, a layer of simple squamous epithelium with underlying connective tissue, moistened with the small amount of fluid within the cavity of the sac.
The abdominopelvic cavity is somewhat arbitrarily divided into the abdominal and pelvic cavities. The abdominal cavity contains the kidneys, most of the digestive organs, and a variable amount of the internal reproductive organs in both sexes. The pelvic cavity contains the terminal part of the digestive system (the rectum) and all of the internal portions of the urogenital system not found in the abdominal cavity. The abdominal and pelvic cavities are continuous with one another, and the brim of the pelvis marks the transition. The serous membrane that surrounds the abdominal viscera and part of the pelvic viscera is called peritoneum.
Figure 1-8. Cross-section of the body wall and digestive tract.
A transverse section through the abdominal cavity illustrates the general plan of the body as a tube (the digestive tract and its derivatives) within a tube (the body wall) (Fig. 1-8). Normally there are few air-filled spaces in the animal body except in the respiratory system and the ear. However, for the sake of clarity, many illustrations show a considerable separation between structures that in the animal body are actually in contact.
The layers of the body wall and the layers of the digestive tract show a striking similarity, although in reverse order. Layers of the body wall from outside inward are (1) epithelium (epidermis of the skin), (2) connective tissue (dermis and fascia), (3) muscle (striated), (4) connective tissue (transverse fascia), and (5) mesothelium (parietal peritoneum). The layers of the gut wall from outside inward are (1) mesothelium (visceral peritoneum), (2) connective tissue (subserous connective tissue), (3) muscle (smooth), (4) connective tissue (submucosa), and (5) epithelium (mucous membrane) (Fig. 1-8).
The serous membranes mentioned previously (pericardium, pleura, and peritoneum) are all derivatives of the lining of the celomic cavity of the embryo. Each serous membrane forms a continuous sac that is usually empty except for a small amount of serous (watery) fluid. In other words, no viscera are found inside any of the serous sacs, although most viscera are covered by at least one layer of a serous membrane. A simple analogy is that of pushing one’s fist into a partially inflated balloon. The fist is never actually within the balloon proper, but still it is surrounded by a portion of the balloon (Fig. 1-9).
Figure 1-9.A) Invagination of serous membrane to form outer (parietal) and inner (visceral) layers. B and C) This is similar to a fist pushed into a balloon.
The part of the serous membrane covering a viscus is called the visceral serous membrane (visceral pericardium, visceral pleura, and visceral peritoneum). The serous membrane lining a body cavity is called the parietal serous membrane (parietal pericardium, parietal pleura, and parietal peritoneum). The continuity of each serous sac is maintained by connecting layers of serous membrane that extend from the visceral layer of each serous membrane to the parietal layer of the same serous membrane. The names of these connecting layers of serous membranes are based on the specific areas they connect, and they are discussed in some detail along with the relevant systems later in this book.
Chapter 2
ANATOMY AND PHYSIOLOGY OF THE CELL
Discovery of living cells would have been difficult, if not impossible, before Zacharias Jansen of the Netherlands invented the compound microscope in 1590. Robert Hooke of England used the term cell to describe the cavities he saw in sections of cork. In 1665, Hooke published a description of cork cells based on a study done with his improved compound microscope.
In 1839 Matthias Schleiden, a German botanist, and Theodor schwann, an animal anatomist, formulated the cell theory, which set forth the concept that “the elementary parts of all tissues are formed of cells in an analogous, though very diversified, manner, so that it may be asserted that there is one universal principle of development for the elementary parts of organisms, however different, and that this principle is the formation of cells.”
The word cell comes from the Latin cella meaning small chamber. In biology, particularly animal biology, the term cell refers more specifically to the individual units of living structure rather than the compartments that may contain them. There actually are no compartments as such in most tissues (with the exception of bone and cartilage), but the living units, cells, are found in groups in which mainly adjacent cells restrain individual cells. As early as 1772, Corti observed the jellylike material in the cell that later was called protoplasm.
Properties of Life
It is difficult to give a satisfactory definition of life. However, the cell is the functional unit of all animal life. It is the unit that makes up all tissues, organs, and systems, which in turn make up the total animal. Therefore, the properties of the cell are equated with those of life. These properties include homeostasis, growth, reproduction, absorption, metabolism, secretion, irritability, conductivity, and contractility. The last two characteristics, however, are not properties of all cells. Conductivity is an important functional characteristic of both nerve and muscle cells, whereas contractility is a property of muscle cells.
Homeostasis is the tendency for living things to attempt to maintain a state of relative stability. At the whole-animal level or at the cellular level, all living things respond to stresses placed upon them by changes in their environment. Their responses are attempts to maintain a state of homeostasis.
Growth is increase in size. Increase in size of a cell or organ beyond normal is called hypertrophy. An increase in the size of a structure due to an increase in the number of cells is called hyperplasia. A decrease in size from normal is called atrophy. Failure of a tissue or organ to develop is called aplasia, while incomplete development or defective development of a tissue or organ is called hypoplasia.
Reproduction of a cell or of an organism implies the ability to produce more cells or more organisms that are essentially the same as the original. Some fully differentiated cells, for instance nerve cells, do not normally retain the ability to reproduce in the adult.
Cells may be found in solutions whose composition is quite different from that of the fluid within the cells. To maintain intracellular homeostasis in these conditions, the passage of particles and water in and out of the cell must be regulated. Absorption is the process of taking dissolved materials or water through the cell membrane into the substance of the cell. This can be a passive process dependent on the forces of diffusion and osmosis, an active process requiring the expenditure of energy from adenosine triphosphate, or the result of electrochemical ionic forces and affinities that require no direct expenditure of energy. All three can occur at the same time across the same cell membrane.
Endocytosis is another means by which extracellular materials may enter a cell. In endocytosis the exterior cell membrane moves to surround extracellular materials in a membrane pocket (Fig. 2-1). This membrane vesicle detaches from the inner surface of the cell membrane and moves into the interior of the cell.
Figure 2-1. Endocytosis (phagocytosis and pinocytosis) and exocytosis.
If a large amount of particulate material is endocytosed by ameboid movements of a cell, the process is more specifically termed phagocytosis (Fig. 2-1), and cells capable of taking in large amounts of material are called phagocytes. This ability is characteristic of some white blood cells, which engulf large particulate matter, tissue debris, or bacteria. After a phagocytic vesicle enters the substance of a cell, it may fuse with a different type of membrane vesicle, a lysosome that was produced within the cell. Lysosomes are specialized membrane vesicles that contain enzymes, also produced within the cell. This fusion permits the lysosomal enzymes to act upon the contents of the phagocytic vesicle in a small, local area that is isolated from the cytosol. Most types of cells are capable of endocytosing small amounts of fluid containing dissolved particles. This type of endocytosis is termed pinocytosis (Fig. 2-1).
Metabolism refers to the sum total of the physical and biochemical reactions occurring in each cell and therefore in the entire animal. Reactions that build and maintain cellular components are called anabolic, and those that break down cellular components or constituents are called catabolic. The oxidation of carbon compounds to carbon dioxide and water, with the release of energy, is a catabolic reaction.
The secretion of products synthesized by the cell into the extracellular fluid (ECF) that surrounds the cells occurs by exocytosis (Fig. 2-1), which is essentially the opposite of endocytosis. Membrane-bound secretory vesicles containing substances synthesized within the cell and packaged by the Golgi apparatus migrate in the cytoplasm to the plasma membrane.
Here the membrane of a secretory vesicle fuses to the exterior cell membrane, an opening appears at the point of fusion, and the contents of the vesicle are released into the ECF.
Irritability (also called excitability) is the property of being able to react to a stimulus. The reaction must necessarily consist of one of the other properties of protoplasm, such as conduction, contraction, or secretion.
Conductivity is the property of transmitting an electrical impulse from one point in the cell to another. This property is discussed in more detail later in this chapter. Nerve cells and muscle cells are specialized for conductivity and irritability.
Contractility is the ability to shorten in one direction. Muscle cells are specialized for contractions, although many other cells and cell organelles also contain contractile proteins and exhibit limited movement (e.g., cilia).
Chemical Composition of the Cell
Chemical composition of various parts of the cell plays an important role in cellular function. The approximate composition of protoplasm by constituent is water, 85%; protein, 10%; lipid, 2%; inorganic matter, 1.5%; and other substances, including carbohydrates, 1.5%.
Water
Each cell is about 60–65% water. Water is by far the largest constituent of protoplasm, which is largely a colloidal suspension in water. Water acts as a solvent for inorganic substances and enters into many biochemical reactions.
Most body water is within cells, and this fluid volume is called intracellular fluid. This fluid volume is about 40% of body weight. The remaining fluid (about 20% of body weight), found outside of cells, is termed extracellular fluid (ECF). Most ECF (about 15% of body weight) surrounds cells throughout the body and is termed interstitial fluid. Unique interstitial fluids include the cerebrospinal fluid, the fluid in the joints, the fluid in the eyes (aqueous and vitreous humors), and the serous fluid in the visceral spaces (i.e., pericardial, pleural, and peritoneal spaces). Blood plasma, a specific type of ECF, is about 5% of total body weight. The percentages for the different types of body fluids vary from one animal to another. Factors affecting these percentages include condition (amount of fat), age, state of hydration, and species.
Water is constantly lost from the body, and it must be replenished if the animal is to remain in water balance and not become dehydrated. Most is lost via the urine, but it is also lost in the feces and by evaporation from body surfaces, such as the skin and respiratory passages. Water replacement is almost entirely by drinking, because minimal amounts of water are produced in the bodies of domestic animals as a result of cellular metabolism (metabolic water).
Proteins
After water, proteins are the next largest constituent of protoplasm. Proteins are complex high-molecular-weight colloidal molecules consisting primarily of amino acids that are polymerized (joined) into polypeptide chains (Fig. 2-2A). The union of amino acids within a protein molecule is by way of a peptide linkage, a bond between the amino (NH2) group of one amino acid and the carboxyl (COOH) group of another amino acid, with the elimination of water. A small chain of amino acids is called a peptide. A polypeptide is a chain of more than 50 amino acids connected by peptide linkages, and a chain that contains more than 100 amino acids is called a protein.
Figure 2-2.A) A chain of amino acids joined by peptide bonds to form a protein. B) A large protein. Each filled circle represents a single amino acid. Chemical bonds between amino acids at distant points in the chain produce the three-dimensional shape of the protein molecule.
The peptide linkages between amino acids in a protein are somewhat flexible, and this permits the chain to bend into various threedimensional shapes (Fig. 2-2B). These configurations may become relatively stable, because chemical attractions, or bonds, form between amino acids at various points in the chain. The three-dimensional shape of a protein is an important determinant of its biologic function, because the shape can determine what segments of the protein chain are exposed and available to interact with other molecules.
Amino acids, and thus proteins, contain carbon, hydrogen, oxygen, and nitrogen. Proteins may also contain other elements such as sulfur, phosphorus, or iron. Simple proteins yield only amino acids or their derivatives upon hydrolysis. The simple proteins, and examples of each, are as follows:
Conjugated proteins consist of simple proteins combined with a component that is not a protein or amino acid. Rather, it is called a prosthetic group. The conjugated proteins and examples of each are as follows:
Most proteins can be classified as structural proteins or as reactive proteins. Structural proteins include these fibrous proteins: collagens, which are the major proteins of connective tissue and which represent about 30% of the total protein content of the animal body; elastins, which are present in elastic tissues such as the ligamentum nuchae, the abdominal tunic, and some arteries; and keratins, which are the proteins of wool, hair, horns, and hoofs. Reactive proteins include enzymes, protein hormones, histones associated with nucleic acids in the nucleus of cells, and contractile proteins in muscle (actin and myosin). Many varieties of proteins are found in blood plasma. Functions of plasma proteins include the transport of substances such as hormones and lipids in the blood, contributing to the process of blood coagulation, and creating an effective osmotic pressure difference between the plasma and interstitial fluid. Plasma proteins also include antibodies, which are produced by certain blood cells and are part of an overall immune response.
All cell membranes contain proteins, and like plasma proteins, the proteins in cell membranes have a variety of functions. These include serving as membrane receptors for hormones and drugs, contributing to the transport of water and particles into and out of cells, acting as membrane-bound enzymes, and serving as markers to permit the immune system to recognize cells as normal or abnormal body components.
Differences in the sequence of the amino acids of the polypeptide chains of proteins often occur between species. For example, the serum albumin in the blood plasma of horses is different from that in the plasma of cattle and sheep. In cattle, the protein hormone insulin is slightly different from that in swine. Such variable proteins may still function in a different species, though usually at levels below that of the naturally occurring form of the molecule. Note: Throughout the text, clinical extracts
