Physiology of farm animals

Front matter

Cornell University Library

The original of this book is in the Cornell University Library.

There are no known copyright restrictions in the United States on the use of the text.

Preface

This book is addressed primarily to students of agriculture who may wish to obtain some knowledge of the simpler physiological processes as they occur in farm animals. It is hoped that it may be of use also to veterinary students. The second volume, by Professor Wood, dealing with animal nutrition, will be published shortly.

I take this opportunity of expressing my indebtedness to all those who have co-operated with me. Professor Wood has kindly read the proof sheets and made various useful suggestions. Mr E. T. Hainan, besides contributing the chapter on the sense organs, has also read the proof sheets and given me the benefit of his criticism ; moreover, all those drawings which are new are the work of his hand. Mr R. H. Adie and Lieut-Colonel W. A. Wood have read the chapter on the locomotor organs and given me the advantage of their special knowledge. I wish to offer my grateful thanks also to those authors and publishers who have allowed me to reproduce illustrations from other works; in particular I desire to mention Sir Edward Sharpey Schafer and Messrs Longmans, Green and Co. I am under much obligation to Major-General F. Smith for the use I have made of his Manual of Veterinary Physiology, published by Messrs Bailliere, Tindall, Cox and Co. Lastly I wish to thank my friend and colleague, Mr K. J. J. Mackenzie, for his constant encouragement and kindly interest, and for the ready help which he has always placed at my disposal.

November, 1919 a 3

Contents

PAGE

I. Introduction ...... 1

II. Histology ....... 13

Introduction

Physiology is the branch of science which is concerned with the functions performed by living things. It is a department of Biology or the science of life. But just as Biology is divisible into Botany which treats of plants, and Zoology which deals with animals, so there are also a Vegetable Physiology and an Animal Physiology, and it is the latter of these which forms the basis of medical and veterinary knowledge. For Physiology is the science which teaches us the way in which the body is built up, and how each part of it works, and the laws which govern its activities. In so doing it helps us to regulate these activities, and by teaching us about the normal working of the body, it shows us how any of its parts may be adjusted when they are out of order. This statement applies to the bodies of animals as well as to those of ourselves.

The present volume deals especially with the Physiology of the domestic animals. This is a branch of the subject which has been much neglected, for notwithstanding the great industrial importance of stock breeding and rearing, it is a business to which physiological science has been applied but little.

Like Chemistry and Physics, Physiology is founded on observations and experiments. These can sometimes be carried out upon ourselves, but are more often made upon animals. By means of a clinical thermometer we can perform a series of observations upon our own bodily temperature, establishing the fact that under normal conditions our temperature remains constant and is independent of that of the outside air which is usually considerably cooler. In this way we can learn a little about the fundamental nature of the organism, since we are led to perceive that our bodies like those of other animals must be supplied with the sources of that form of energy which we recognise as bodily heat. By extending our observations, and noting the increased blood supply of the skin and the tendency to sweat when we feel warm, we learn something about the heat regulating mechanism which we possess in common with other warm-blooded animals. Again, by taking advantage of the fact that all the parts of an animal do not die at the same time we can perform a great variety of experiments which teach us much about the functions of the different organs of the body. Thus the excised limb muscle of a frog if kept moist by a saline solution, or the heart of a mammal if perfused with an artificial fluid resembling blood serum and preserved at body temperature, may be induced to survive many hours after the death of the animals from which they were taken, and from experiments upon these it is possible to learn much concerning the mechanisms of muscular contraction and the heart beat. Furthermore, we can carry out experiments upon entire animals, pain being avoided by the administration of anaesthetics during the progress of the operation.

Many of the functions of the body are essentially similar throughout the whole animal kingdom, and among Vertebrates there is a still closer likeness. Thus the processes of digestion in a frog, a bird, a rabbit, a horse, and a man are broadly speaking similar, though there are of course very marked differences which are generally greater the less closely related the animals are. So also the general laws governing the nature of nerve impulses, the movements of the heart, or the processes of reproduction are identical in every case. Thus by investigating any of these processes in a frog or in a rabbit, for example, we can learn a great deal about the functional activities of a horse, or a sheep, or a cow ; and it is desirable that before specialising upon the Physiology of farm animals we should possess a general knowledge of the principles of the science obtained by studying whatever animals are most convenient for this purpose.

Furthermore, Physiology must always be studied in close relation to Anatomy or Morphology (that is, the department of Biology which deals with the form and structure of organisms), since it is impossible to accpiire an insight into the functions of the various parts of an organism without possessing some knowledge of the composition and structural relation of those parts; and conversely, an intelligent comprehension of the form and structure of an organ can only be gained by a consideration of the part which that organ plays in the general economy of the individual.

I]

INTRODUCTION

The physical basis of all life, both vegetable and animal, is called protoplasm. The vital substance forming the most highly developed animals differs in degree rather than in kind from the undifferentiated protoplasmic mass composing the most simple form of life known. Protoplasm is a semi-fluid, transparent, viscous substance, which occurs usually in small individual particles, called cells. Sometimes it seems to be quite homogeneous but more often a reticulated structure can be detected. A study of its composition reveals that its most abundant constituent is water, which amounts to 75 per cent, of the whole material. The remaining 25 per cent, is made up of solids which consist chiefly of nitrogenous compounds called proteins, but certain metals (potassium and calcium) as well as phosphorus and sulphur are also present in a combined form together with small quantities of fats and carbohydrates.

An elementary knowledge of the physiology of protoplasm may be gained by studying the vital manifestations of the

O iA A / SO

Fig. 1. Amoeba (after Graves, from Shipley and MacBride). 1 Nucleus, 2 Contractile vacuole through which waste products are excreted, 3 Pseudopodia, 4 Vacuoles containing food, 5 Grains of sand.

amoeba. This is a minute organism found in stagnant water, and resembling, when seen under the microscope, a little lump of moving jelly. It can be observed to move about spontaneously,

4 PHYSIOLOGY OF FABM ANIMALS [CH.

to eat up little particles of food, to excrete or get rid of waste products, to grow in size, and lastly, at a certain stage in its life-history, to reproduce by undergoing a process of simple division into two. In order to perform these functions it is essential that it should receive a supply of food, in just the same way as an engine cannot be made to work unless it is provided with fuel. In the latter case the fuel undergoes combustion which liberates heat. So also in the case of the animal the energy is derived from the complex food material, which undergoes a process of slow oxidation, thereby breaking down into simpler substances and setting free the energy necessary for the discharge of the vital functions. The changes which the food or its constituents undergo in the amoeba or any other organism « are classed together under the term 'metabolism': those of them which relate to the building up of the food into the material of the body are referred to as ' anabolic ' or ' assimilative' ; while the changes which are associated with activity, resulting from a breaking clown of complex substances into more simple ones, are known as 'katabolic' or 'dissimilative.'

An amoeba consists of a single cell, that is to say, a minute mass of protoplasm containing within it a certain specialised portion known as the nucleus which can generally be readily identified under the microscope by its more intense staining capacity. The nucleus is essential to the life of the cell. There are many different kinds of unicellular organisms, varying in form but all resembling the amoeba in their general plan. Higher in the scale we find groups of amoeba-like cells, each with their nucleus, aggregated together with little or no division of function. Such an arrangement occurs in the simpler kinds of sponges. But in the majority of multicellular animals whole groups of cells are separated off to subserve particular functions ; and these form the various tissues. The body of a higher animal is, however, derived from a single cell essentially similar to that of an amoeba, and this, in the process of individual development, undergoes a long series of divisions in the progress of which the nuclei also divide. The products of division, that is to say, the cells with their contained nuclei, become gradual^ specialised to form the different tissues— bone, cartilage, muscle, nerve, connective tissue, etc. Thus the outer layer of cells becomes adapted for protection and for receiving the impressions produced by

I]

INTRODUCTION

Fig. 2. Diagrammatic longitudinal section through Hydra, a tubular animal showing cellular differentiation of a simple kind. (From Shipley and MacBride.) 1 Mouth, 2 Foot, 3 Tentacle, 4 Digestive Cavity, 5 Outer layer of cells or Ectoderm, 6 Inner layer of cells or Endoderm, V Structureless jelly between the two latter, 8 Groups of cells specialised for offensive purposes and containing structures which can be ejected, 9 Testis, 10 Ovary.

changes in the surroundings ; the inner layer lining the gut becomes adapted for the digestion and assimilation of food ; while between these are developed the skeleton and general framework of the body, and all the other tissues which assist in performing the vital functions.

These functions may now be considered more closely. The parts concerned with each function are usually called systems, and the subsidiary parts which compose these systems are known as organs. The following are the principal systems of the body in a higher animal: (1) the digestive system ; (2) the circulatory system; (3) the respiratory system; (4) the excretory system; (5) the muscular system ; (6) the nervous system ; and (7) the reproductive system. In addition to these are the organs of special sense, such as the eye and ear, and the various organs of internal secretion.

Digestive System. In such lowly forms of life as the amoeba, food is taken in at any point on the surface, and is then assimilated, the indigestible residue being cast out at some other part of the surface. But in man and in all the other higher animals there is a differentiation of function, food being taken in only at a definitely located mouth, whence it passes down an alimentary canal which is divided into several portions. In some of these the digestible material is absorbed, passing through the wall of the canal and into the neighbouring lymph vessels or blood vessels, whence it is distributed throughout the body. The indigestible residue is expelled at a definite anus, which is placed at the hind end of the body. The actual digestive process takes place through the action of various glands such as the salivary glands, the gastric glands, the liver and the pancreas, all of which pour out juices having a digestive or solvent action on the food stuffs.

Circulatory System. To enable the food which is absorbed from the alimentary canal to be distributed to all the organs of the body a circulatory medium is provided, and this medium is the blood. In all higher animals the blood is kept in motion by a central organ of propulsion, the heart. In a mammal the heart is divided into four chambers — a right and a left auricle, and a right and a left ventricle. Each chamber communicates with the adjoining ones by valves, which only permit of the blood passing in one direction. The circulation is maintained by

I] INTRODUCTION 7

the heart alternately contracting and dilating, the compressing force being supplied by the muscular wall of the organ. The heart communicates with an elaborate system of vessels, those which carry the blood away from it being called arteries, while those which bring the blood back are called veins. The arteries possess thicker and more elastic walls than the veins. The arteries become divided up in the peripheral parts of the body into a number of much smaller vessels, the capillaries, which permeate the tissues. Some of the fluid of the blood transudes through the walls of these vessels, becoming the lymph and bathes the cells with blood. The capillaries unite again to form the veins, which transport the blood back to the heart. If the web of a living frog's foot be examined under the microscope, the circulation of the blood in the capillaries may be observed quite easily.

The red colour of blood is due to the presence of innumerable little red disks known as corpuscles, which float in a yellowishcoloured semi-transparent fluid, the blood plasma. In addition to these red corpuscles, blood also contains a relatively small number of white corpuscles or leucocytes, which have the power of independent movement and resemble the unicellular amoebae referred to above. One of their functions is to eat up and so destroy the germs of disease.

As already mentioned some of the products of digestion are absorbed into vessels containing lymph which is a colourless fluid resembling blood plasma. The lymph vessels or lymphatics communicate with veins so that the substances present in lymph eventually enter the blood system.

Respiratory System. In order to keep an animal alive it is necessary to supply it with oxygen, for, as we have seen, every animal is dependent for its source of energy upon the slow oxidation of the material which it builds up out of its food supply. As a result of this oxidation process it is continually giving off carbon dioxide. In the higher vertebrates the organs which are concerned in this gaseous exchange are the lungs ; but in the lower forms of life the entire surfaces of the body serve to effect the interchange of oxygen and carbon dioxide ; while in many other animals, which live in water (e.g. fishes), the respiratory organs take the form of gills.

The lungs of an animal are connected with the exterior by

8 PHYSIOLOGY OF FARM ANIMALS [OH.

the windpipe or trachea, which opens into the back of the mouth at the root of the tongue. This tube at its lower end divides into the two bronchi, and these subdivide again and again within the lungs like the branches of a tree. Their finest divisions widen out into air sacs which are in close relation to a meshwork of capillaries, the air in the air sacs and the blood in the vessels being separated from one another by only a very thin layer of protoplasm. These air sacs and capillaries form an essential part of the structure of the lungs. Here the gaseous exchange is effected, the blood taking up oxygen from the air sacs and giving off carbon dioxide. When the blood passes to the tissues this process is reversed, the carbon dioxide being taken up from the tissue cells, while oxygen is given off to these cells. The arterial blood, or blood which is being circulated to the tissues after having been to the lungs and heart, is bright-red in colour owing to the presence of a compound called oxyhaemoglobin, which contains oxygen in loose combination : whereas venous blood which is in process of being returned to the heart and lungs is purple, the oxyhaemoglobin having been reduced, in part at least, to haemoglobin, thereby causing a change in the colour of the corpuscles. The complete course of the circulation in a mammal is as follows : Right auricle of heart, right ventricle, pulmonary arteries, lungs, pulmonary veins, left auricle, left ventricle, arteries (excepting pulmonary arteries), capillaries, veins (excepting pulmonary veins), and so back to the heart.

The interchange of gases which takes place between the lungs and the external air is effected by the alternate expansion and contraction of the chest wall, the air passing through the windpipe in opposite directions in the acts of inspiration and expiration. In this way the excess of carbon dioxide in the lungs is got rid of, and a supply of oxygen from the external air is able to take its place

Excretory System. We have seen that the protoplasm of which an animal is composed consists principally of protein. This consists of carbon, hydrogen, oxygen, nitrogen, and sulphur. As a result of katabolism these elements must be got rid of. A great part of the waste carbon and oxygen is disposed of by the lungs in the form of carbon dioxide as just described. The hydrogen and a further proportion of oxygen are excreted in the form of water vapour both through the lungs and the

Ij INTRODUCTION 9

skin. The waste nitrogen, however, and also the waste sulphur (but this is only a small amount, since sulphur is present in only small quantity in protoplasm) are got rid of for the most part by the kidneys, which are commonly described as excretory organs. For just as the cells of the body discharge their waste carbon dioxide into the blood, so also do they dispose of their nitrogenous products, and these are carried to the kidneys. In the latter the blood capillaries are separated by only a single cellular layer from the cavity which communicates with the exterior. The cells of this layer absorb the waste products and excrete them into the kidney tubules, together with water and some salts in solution. These form the urine, which flows down a duct termed the ureter into the bladder, from which it is expelled at intervals to the exterior in the act of micturition.

Muscular System. A muscle is an especially contractile organ which is either circular (as in sphincter muscles) or straight. The latter is the more usual form among the higher animals. Such a muscle, upon contracting, reduces the length between its two farthest points. One of these points is usually called the origin of the muscle and the other its insertion. The muscles are attached to bones, and these by their movements may become inclined to one another at various angles. In the case of the limbs these angles are opened and closed, thereby causing progression, and the mechanical aid which is introduced to effect this is that of the lever. The nuiscles themselves are fleshy masses composed of fibres. Some muscles, such as those of the limbs, are under the direct control of the will and are consequently often called voluntary muscles; whereas others like the muscles surrounding the intestines, which are not under the control of the will, are termed involuntary muscles. The voluntary muscles form a considerable part of the body, and constitute the flesh or meat on an animal. Broadly speaking, the muscular system is that part of the body in which the energy set free by the oxidation of the food material is converted into motion and so carries on work.

Nervous System and Sense Organs. In the lowest forms of animal life the protoplasm is uniformly irritable and contractile ; but in the higher types, as just shown, the organs of movement are concentrated in a motor or muscular system, while there are also definite organs of sense (eye, ear, etc.). Such division of labour necessitates a means of communication between the various organs, and this function is fulfilled by the nervous system. The nerves consist of strands of a peculiar kind of tissue constituting the nerve fibres, and these connect together the sense organs and muscles and all the various parts of the body. The nervous system is presided over by the brain and spinal cord. The former of these is contained within the skull, while the latter consists of a hollow tube enclosed by the backbone or vertebral column. The brain and spinal cord together constitute the central nervous system. The other nerves are differentiated into afferent (sometimes called sensory) nerves, in which the impulses pass from the sense organs to the central nervous system, and efferent (sometimes called motor) nerves, in which the impulses travel outwards from the central nervous system to the muscles or other organs to which a message is to be sent. All actions which are under the control of the will are presided over by the brain, from which the voluntary nervous impulses are transmitted. But there is another class of actions in which either the brain or spinal cord is concerned, but which are involuntary. These are called reflex actions. For example, if a frog after being deprived of its brain be hung up by its jaw and one of its toes pinched, its leg is drawn up. Such an act is involuntary, and in this case is controlled by that part of the central nervous system which is still intact, namely the spinal cord. If this be destroyed the reflex action can no longer take place. It is clear, therefore, that such an act involves a succession of processes, which are as follows. The pinching of the toe supplies a stimulus which is transmitted as a nerve impulse by an afferent nerve to the spinal cord. Another nerve impulse is then transmitted by an efferent nerve in the opposite direction, passing from the spinal cord to the muscles of the limb, and this causes them to contract. There are also cerebral reflexes, which are likewise involuntary. For example, the secretion of the saliva and the secretion of the gastric juice are reflexes which are induced by the introduction of food into the mouth, messages being transmitted in the first place in an afferent direction from the mouth to the brain, and then in an efferent direction from the brain to the secretory glands of the mouth and stomach. It is interesting to note that different sets of nerves are concerned in transmitting the impulses in opposite directions. Every reflex

I] INTRODUCTION 1 1

is presided over by a special centre in the central nervous system, these centres being the parts of the brain and spinal cord that receive the afferent impulses and dispatch the efferent ones which are concerned in the reflexes in question. In the body of a higher animal a close succession of reflex actions is continually going on, and it is in this way that the individual organism is able to react to environmental changes, and so fit itself to its surroundings.

Reproductive System. In the unicellular organisms like the amoeba, reproduction is carried on by simple cell division. In the higher animals, however, there are certain special cells set apart for reproduction, the ova in the female and the spermatozoa in the male. The ova and spermatozoa are produced respectively in the ovaries and testicles. Their function is to unite together, a single ovum fusing with a single spermatozoon and giving rise to a conjugated cell or oosperm, which by a long succession of cell divisions develops into a new individual. In each of these divisions the cell nuclei also divide so that each product of division always contains a nucleus. In the mammal the development of the unborn young takes place in a special organ, the uterus, ' womb ' or ' bed ' whence they are expelled in the act of parturition or giving birth.

Organs of Internal Secretion. In addition to the various systems described above, we find also certain organs that have the power developed to a special degree of altering the composition of the blood by secreting into it chemical substances which are elaborated for the advantage of other parts of the body, whither they are conveyed in the blood stream. Many of the organs referred to above which subserve the functions already mentioned are in addition internally secreting glands, thus fulfilling more than one purpose. Such organs are the liver, the pancreas, the ovary, and the testicle. The liver, besides secreting bile, stores up a supply of carbohydrate in the form of glycogen, and when required liberates it into the blood as sugar. The pancreas also in some unknown way controls the carbohydrate metabolism, since after its experimental removal sugar makes its appearance in the urine, thereby indicating an excess of sugar in the blood. But there are certain other organs, which appear to be solely organs of internal secretion. Such are the suprarenal bodies, which are situated just in front of the kidneys, one

12 PHYSIOLOGY OF FARM ANIMALS [CH. I

on either side. These secrete into the blood a substance known as adrenalin, which acts on the muscles, particularly those of the blood vessels. If these organs are removed extreme weakness, associated with muscular collapse, results, and is followed sooner or later by death.

Chapter II

Histology is the study of the minute structure of the tissues of the body. Since it is carried out by means of the microscope it has been described as Microscopic Anatomy.

The tissues of which the different organs are composed may be divided into four main groups as follows :

(1) Epithelial tissues.

(2) Connective tissues.

(3) Muscular tissues

(4) Nervous tissues.

(1) Epithelial Tissues.

An epithelium may be defined as a cellular membrane bounding a free surface. There is a certain amount of cementing substance present between the cells, but this is reduced to minimal proportions. There are two main kinds of epithelia, simple and compound.

In simple epithelia the cells form one layer only. They may be cubical in shape like those lining the tubules of certain glands such as the pancreas, or columnar like some of the cells lining tfininmm iLmi± i imiiM ^ l J"+ l i > <i ^

Fig. 3. Columnar epithelium from intestinal villus (from Gray).

the inside of the stomach or intestine. They may be provided with cilia or fine filaments, as with the cells of the uterus, in which case we speak of a ciliated epithelium. Moreover the lining membrane may consist of a layer of thin cells arranged in the

J4

[CH.

manner of flat paving-stones closely fitted together such as we find surrounding the air sacs of the lungs. Such a tissue is described as a pavement epithelium.

Fig. 4. Ciliated epithelium from trachea (from Gray), a elastic fibres within, h, c, d epithelial cells of mucous membrane, e ciliated epithelial cells.

Fig. 5. Transitional epithelium (from Gray).

Fig, 6. Stratified epithelium from oesophagus (from Gray).

Iii compound epithelia the tissue consists of more than one layer of cells. When the cells are arranged in two, three or four superimposed layers the epithelium is frequently called transitional, but if the number of layers is considerable, we speak of a stratified epithelium. The tissue lining the urinary bladder is n]

a transitional epithelium ; that forming the epidermis or superficial portion of the skin is a stratified epithelium. In addition to these kinds of epithelia there are the more specialised forms

Fig. 7. Diagram illustrating development of different kinds of glands (from Gray). A simple gland, B sacculated gland, C convoluted tubular gland, I) and E racemose glands, F compound tubular gland.

present in secreting glands, and certain sense organs, besides those structures like horn, tooth enamel, etc. which are of the nature of modified epithelia. These are described later in dealing with the physiology of the organs concerned.

(2) Connective Tissues.

The connective tissues are found throughout the whole body lying between and binding together the different organs or parts of the same organ. Their function is to act in a purely mechanical manner giving support where required and at the same time

[CH.

admitting of the necessary amount of elasticity or rigidity. In embryonic development they have an identical origin, and there are all gradations between the various kinds of connective tissue. They agree further in having a large amount of intercellular cementing substance, and in this substance fibres are developed. Although situated outside of the cells this intercellular substance in the first instance was derived from the cells.

One of the commonest kinds of connective tissue is called areolar tissue, which is found in great abundance just under the skin. It consists largely of a close meshwork formed of bundles of fibres which^are white in colour and very fine and wavy.

Fig. 8. Areolar tissue (after Szymonoviez from Halliburton), 1 branched cell, 2 white blood corpuscle emigrated from a neighbouring vessel, 3 elastic fibres, 4 white fibres.

Elastic fibres are also present. These are generally thicker than the white fibres ; they are yellowish in colour, and as a rule much straighter. In addition to the fibres there is a clear ground substance containing several kinds of cells which can be seen lying amid the fibres. The cells, which contain easily discernible nuclei, are in many cases flattened and branched, but there are other kinds of cells which resemble or are identical with some of the white corpuscles of the blood.