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 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).