CHAPTER I

How the Animal Body is Formed

The cell is the unit of growth. It is so with all forms of life--plant or animal, insect or bacterium. In the beginning the start is with a single cell, an egg, if you please. After fertilization has taken place, this single cell enlarges or grows. Many changes now occur, all rather rapidly, until the cell walls become too small, when it breaks apart and forms two cells just like the first used to be. This is known as cell division. As growth increases, the number of cells increases also--until in the end there are millions.

=Nature of the Cell.=--The cell is very small. In most cases it cannot be seen with the naked eye. The microscope is necessary for a study of the parts, the nature and the character of the cell.

In the first place the cell is a kind of inclosed sac, in which are found the elements of growth and life. Surrounding the cell is a thin wall known as the cell membrane. In plants this cell wall is composed of cellulose, a woody substance, which is thin and tender in green and growing plants, but hard and woody when the plant is mature.

Within the limits of the cell is the protoplasm, the chief constituent of the cell; locked up in this protoplasm is life, the vital processes that have to do with growth, development, individual existence.

Embedded within the protoplasm is another part known as the nucleus and recognized under the microscope by its density. Around the nucleus is centered the development of new cells or reproduction--for the changes that convert the mother-cell into offspring-cells are first noted in this place.

So much for plant cells. Is this principle different in animals? For a long time it was thought that plants and animals were different. But upon investigation it was discovered that animals were comprised of cells just as plants. And not only was this discovered to be true, but also that animal cells corresponded in all respects to plant cells. Hence in animals are to be found cells possessing the cell walls formed of a rather thick membrane, the granular protoplasm or yoke, and the nucleus established in the yoke.

The ovum, known as the female egg, is composed of the parts just described. If it is not fertilized when ripe it passes away and dies. If fertilized in a natural way, it enlarges in size and subsequently divides into two cells; and these, passing through similar changes, finally give rise to the various groups of cells from which the body is developed.

=The Animal Body a Group Collection.=--The body is, therefore, a mass of cells; not all alike, of course, but grouped together for the purpose of doing certain special kinds of work. In this way we have various groups, with each group a community performing its own function. The brain forms one community; and these cells are concerned with mind acts. The muscle cells are busy in exerting force and action. Another group looks after the secretions and digestive functions, while another group is concerned solely with the function of generation and reproduction. And so it is throughout the body.

Both individual cells and group cells are concerned with disease. One cell may be diseased or destroyed, but the surrounding ones may go on just the same. It is when the group is disturbed that the greatest trouble results.

=A Word About the Cells.=--The cell always possesses its three parts--membrane, protoplasm, and nucleus. But there is no rule as to the size or shape. Cells may be round or oblong, any shape. Substances pass in and out of the cell walls; and they are in motion, many of them, especially those that line the intestines and the air passages, and the white corpuscles of the blood. More than this, some cells, Dr. Jekyl-like, change their appearance and shape, send out finger-like bodies to catch enemies or food, and even travel all around in the body, often leaving it altogether.

BODY TISSUES

The animal body contains five forms of tissues: Epithelial, in which the cells are very compact, forming either thin or thick plates; the connective tissue, by which many organs are supported or embedded; muscle tissue, either smooth or striated, and in which the cells are in fibers that contract and shorten; nerve-tissue, that has to do with nerve and ganglion cells by which mental impulses are sent; and blood and lymph tissue or fluid tissues.

The first group is intimately connected with the secretory organs, or those organs which secrete certain substances essential for the proper work of the body. Thus we have salivary glands, mucous glands, sweat glands, and the liver and pancreas. Connective tissue includes fibrous tissue, fatty tissue, cartilage and bone. The fibrous connective tissue is illustrated when the skin is easily picked up in folds. Fatty tissue occurs where large amounts of fat are deposited in the cells. Cartilage is found where a large amount of firm support is required. With muscle we are all familiar; it is the real lean meat of the body.

=Blood and Lymph.=--The blood is a fluid in which many cells are to be found. The fluid is known as serum or blood-plasma and the cells as corpuscles, and are both red and white. The red cells give the characteristic color. When observed under a microscope, they appear as small, round disks. They are of great importance to the body work. Because of the coloring matter in them the oxygen of the air is attracted when it comes in contact with the blood in the lungs. Oxygen is in reality absorbed, and on the blood leaving the lungs it is distributed to all parts of the body. The oxygen supply of the body is, therefore, in the keeping of the red corpuscles.

White corpuscles have a different work; they guard the body by picking up poison, bacteria, and other undesirable elements and cast these out through the natural openings of the body. Compared with the red cells, they exist in far less numbers and may wander about through all parts of the body.

Lymph is a fluid in which a few cells, lymph corpuscles, are suspended. These cells are very much like the colorless corpuscles of the blood, only no red blood cells are present. But the lymph attends to its own business; it bathes the tissues and endeavors to keep them in a healthy condition.

=Skin and Hair.=--Without a covering the delicate muscles would be unprotected. The skin serves in this capacity. It does still more; out of it is exuded poisonous substances, perspiration, and, at the same time, the skin is a sort of respiratory organ, through which much of the carbonic acid formed in the body escapes.

The skin possesses two general layers, the cutis and sub-cutis; in the first is contained also epidermis. Developed in the skin are the outer coverings like hair, wool, feathers, horns, claws, and hoofs.

THE FRAMEWORK OF THE BODY

The framework of the body undergoes a gradual development from birth to maturity. It represents the bony structure of the body; and on it all other parts depend for support and protection. The brief summary of its parts and work that follows here has been adapted from Wilcox and Smith.

=The Skeleton.=--This consists of a backbone, skull, shoulder girdle, pelvic girdle, and two pairs of appendages. The backbone may be conveniently divided into regions, each comprising a certain number of vertebræ. The cervical vertebræ include those from the skull from the first rib. In all mammals except the sloth and sea cow the number of cervical vertebræ is seven, being long or short, according as the neck of the animal is relatively long or short. The first and second cervical vertebræ, known as the atlas and axis, are especially modified so as to allow free turning movements of the head.

The next region includes the dorsal or thoracic vertebræ, which are characterized by having ribs movably articulated with them. The number is 13 in the cat, dog, ox, sheep, and goat; 14 in the hog; 18 or 19 in the horse and ass, and six or seven in domestic poultry. In mammals they are so joined together as to permit motion in several directions, but in poultry the dorsal vertebræ are more rigidly articulated, those next to the sacrum often being grown together with the sacrum. The spines are high and much flattened in all ungulates, long and slender in dogs and cats. They slope backward, forming strong points of attachment for the back muscles. Several ribs, varying in number in different animals, meet and become articulated with the breast bone or sternum. The sternum consists of seven to nine articulated segments in our domestic mammals, while in fowls the sternum is one thin high bone furnished with a keel of varying depth. The lumbar vertebræ lie between the dorsal vertebræ and the sacrum. The number is five in the horse, six in the hog, ox and goat, and seven in the sheep. The sacrum is made up of a certain number of vertebræ, which are rigidly united and serve as an articulation for the pelvic arch. The number of sacral vertebræ is five in the ox and horse, four in sheep and hogs, and 12 to 17 in birds. The caudal or tail vertebræ naturally vary in number according to the length of the tail (7 to 10 in sheep, 21 in the ox, 23 in hogs, 17 in the horse, 22 in the cat, 16 to 23 in the dog).

In ungulates the anterior ribs are scarcely curved, the chest being very narrow in front. The number of pairs of ribs is the same as the number of dorsal vertebræ with which they articulate.

=The Skull.=--This part of the skeleton is really composed of a number of modified vertebræ, just how many is not determined. The difference in the shape of the skulls of different animals is determined by the relative size of the various bones of the skull. In hogs, for example, the head has been much shortened as a result of breeding, thus giving the skull of the improved breeds a very different appearance from that of the razorback.

The shoulder girdle consists of a shoulder blade, collar bone and coracoid on either side. The fore leg (or wing, in case of birds) articulates with the socket formed by the junction of these three bones. In all the ungulates the shoulder blade is high and narrow, the coracoid is never much developed, and the collar bone is absent. In fowls all three bones of the shoulder girdle are well developed, the collar bone being represented by the “wish bone.”

=The Pelvic Girdle.=--This consists of three bones on either side, viz., ilium, ischium, and pubis. The first two are directly articulated to the spinal column, while the pubic bones of either side unite below to complete the arch. The three bones of each side of the pelvis are present in all our domestic animals, including the fowls.

=Legbones of Farm Animals.=--There is one formula for the bones of the fore and hind legs of farm animals. The first segment is a single bone, the humerus of the fore leg, femur of the hind leg. In the next segment there are two bones, radius and ulna in the fore leg, tibia and fibula in the hind leg. In the dog, cat, and Belgian hare the radius and ulna are both well developed and distinct. In ungulates the humerus is short and stout, while the ulna is complete in the pig, rudimentary and behind the radius in ruminants and firmly united with the radius in the horse. Similarly with the hind leg the fibula is a complete bone in the pig, while in the horse there is merely a rudiment of it, attached to the tibia.

=Feet.=--The mammalian skeleton has undergone the greatest modification in the bones of the feet. In the horse there are only six of the original ten wrist or carpal bones, and, since there is but one of the original five toes, the horse has also but one metacarpal or cannon bone. Splint-like rudiments of two other metacarpal bones are to be found at the upper end of the cannon bone, or at the “knee” joint. Below the cannon bone, and forming the shaft of the foot, we have the small cannon bone, coronary bone, and coffin bone--the last being within the hoof with the navicular bone behind it. The stifle joint of the horse corresponds to the knee of man. The “knee” of the horse’s fore leg corresponds to the hock of the hind leg, both being at the upper end of the cannon bone. The fetlock joint is between the large and small cannon bones, the pastern joint between the small cannon or large pastern bones, and the coffin joint between the coronary and coffin bones. The horse walks upon what corresponds to the nail of the middle finger and middle toe of man.

In pigs four digits touch the ground, the first being absent and the third and fourth larger and in front of the second and fifth. In ruminants the third and fourth digits reach the ground, while the second and fifth do not. In dogs the first digit appears on the side of the leg, not in contact with the ground.

In fowls the wing, which corresponds to the fore leg of mammals, shows a well-developed humerus, radius and ulna, while only one carpal and one metacarpal bone remain, along which the wing feathers are attached. In the leg the femur and tibia are strong bones, but the fibula is a mere splint. The tarsal bones are absent, while the shank consists of a metatarsal bone (really three bones fused together), to which the four toes are articulated.

=The Muscular System of Farm Animals.=--The muscular system is too elaborate, the number of muscles too great, and their modifications for different purposes too complex for consideration in detail in the present volume. All muscles are either striped or unstriped (as examined under the microscope), according as they are under the immediate control of the will or not. The heart muscle forms an exception, for it is striped though involuntary. The essential characteristic of muscle fibers is contractility, which they possess in high degree. The typical striped muscles are concerned in locomotion, being attached at either end to a bone and extending across some movable joint. The most important unstriped muscles are found in the walls of the intestines and blood vessels.

=The Nervous System.=--In so far as our present purposes are concerned, the nervous system may be disposed of in a few words. The central nervous system consists of a brain and spinal cord. The microscopic elements of this tissue are peculiarly modified cells, consisting of a central body, from which fibers run in two or more directions. The cell bodies constitute the gray matter, and the fibers the white matter of the brain and spinal cord. The gray substance is inside the spinal cord and on the surface of the brain, constituting the cortex. The most important parts of the brain are the cerebrum, optic lobes, cerebellum, and medulla. There are twelve pairs of cranial nerves originating in the brain and controlling the special senses, movements of the face, respiration, and pulse rate. From each segment of the spinal cord a pair of spinal nerves arises, each of which possess both sensory and motor roots. The sympathetic nervous system consists of a trunk on either side, running from the base of the skull to the pelvis, furnished with ganglionic enlargements and connected with the spinal nerves by small fibers.

=The Respiratory Organs.=--These include the nose, larynx, trachea or windpipe, and lungs. The trachea forks into bronchi and bronchioles of smaller and smaller size, ending in the alveoli or blind sacs of the lungs. In fowls there are numerous extensions of the respiratory system known as air sacs, and located in the body cavity and also in the hollow bones. The air sacs communicate with the lungs, but not with one another.

=The Urinary Organs.=--These consist of kidneys connecting by means of ureters with a bladder from which the urethra conducts the urine to the outside. In the male the urethra passes through the penis and in the female it ends just above the opening of the vagina. The kidneys are usually inclosed in a capsule of fat. The right kidney of the horse is heart-shaped, the left bean-shaped. Each kidney of the ox shows 15 to 20 lobes, and is oval in form. The kidneys of sheep, goats, and swine are bean-shaped and without lobes.

=The Reproductive Apparatus.=--This consists of ovaries, oviducts, uterus or womb, and vagina in the female; the testes, spermatic cords, seminal vesicle and penis, together with various connecting glands, especially prostate gland and Cowper’s gland, in the male. In fowls there is no urinary bladder, but the ureters open into the cloaca or posterior part of the rectum. The vagina and uterus are also wanting in fowls, the oviducts opening directly into the rectum. The male copulating organ is absent except in ducks, geese, swan, and the ostrich.