=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.
[Illustration: ONE OF THE PARASITES OF THE HOG
The thorn-headed worm attached to the anterior part of the small intestine often causes death. Not more than five or six are usually found in a single animal.]
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.