=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.