on the tips of which the weight of the body is supported. Finally, horses have but a single digit on each foot, the end bone of which is covered with the hoof (Fig. 84). The story of the horse, as it is learned from the fossil Splints of 2nd and 4th digits thing was about the size of a fox, and walked on the distal part of four fingers of each front foot and of three hind toes, all of about equal size. Gradually, however, the descendants of this animal came to walk more and more on the tips of the middle fingers and the middle toes. The little fingers were there fore too short to Three Toes Side toes not touching the ground Three Toes Side toes touching the ground; splint of 5th digit Four Toes Four Toes One Toe Splints of 2nd and 4th digits Three Toes Side toes not touching the ground Three Toes Side toes touching the ground Splint of 1a digit Three Toes Splint of 5th digit. touch ground, they FIG. 86.-The Development of the Fore and became smaller as the ages passed, and have altogether dis Hind Feet of a Horse. Feet of Modern Horse are figured at the Top. - From diagram in American Museum of Natural History. As the middle digit came to appeared in the modern horse. be used more, its size notably increased, while there was a corresponding decrease in the size of the digits on either side. In skeletons of horses that lived nearer modern times, we lose all trace of the phalanges of these two side digits, and in the horse of to-day there is nothing to suggest this long story with the exception of two so-called splint bones along the sides of the lower leg; these are the remains of the two metacarpal bones to which the two side fingers were attached. Peculiarities of the Human Skeleton. - In the human body there is no bone or set of bones that is not found in varied form in all the mammals and in most other vertebrates; indeed so far as we can learn from the structure of his skeleton, man is much more closely related to the gorilla and the chimpanzee than are these animals to the lower monkeys. Yet there are certain general peculiarities of form that are found in the skeleton of man alone, these distinctive characteristics being due in a great degree to his erect position. In the first place, in even the highest monkeys the length of the arms is nearly equal to that of the legs. For while the gorilla can walk on two feet, all four appendages are often employed in locomotion. In man, on the other hand, the legs are much longer than the arms, an advantage that permits of long strides in walking. Again, no other animal has the four curves in the spinal column and the arched instep. These provisions are more necessary in man because the head rests on the top of the spinal column, and any sudden jar would be transmitted to the brain were it not for the presence of these elastic springs. The human skull is nearly balanced on the top of the spinal column, while that of other animals is attached to the anterior end of a more or less horizontal backbone. Man's cranium is much larger than the skeleton of the face, whereas even in the highest monkeys the heavy face bones more than balance the bones of the brain-case, and thus it is difficult for the animal to hold its head erect for any length of time. Finally, the gradual increase in the size of the vertebræ from the neck to the sacrum and the breadth of the pelvis (both characteristics peculiarly human) give a stable base on which the erect trunk is supported by the legs. CHAPTER IX A STUDY OF THE MUSCLES Importance of Muscle Tissue. Muscle tissue constitutes 41% or almost half of the weight of the human body. In this kind of tissue is found one fourth of all the blood. But the importance of muscle tissue is appreciated, even more fully, when we realize that nearly every kind of movement in the body is due to the action of the muscles. Not only do they bring about the more obvious motions of the arms, the legs, the trunk, and the head, but also to muscular action are due all the contractions of the heart, of the stomach, and of the other internal organs. Every change in the expression of the face, and every variation in the tone of the Neck. voice is likewise a result of FIG. 87.- Muscles of the Head and the action of this all-important tissue. Hence we are not surprised that there are over five hundred separate muscles, which vary in length from the fraction of an inch (within the ear cavity) to over a foot and a half (down the front of the thigh). Kinds of Muscle. All of these muscles are in one way or another under the control of the nervous system. Some = of them are directed by the conscious portions of our brain. Thus we can close our fingers and open them as we please; we can move the eyes, the head, and the legs at will. We call all the muscles that are controlled by our will power, vol'un-ta-ry muscles (Latin voluntas will). Most of the muscles of the throat, those of the gullet, stomach, and intestines, on the other hand, act without any voluntary direction on our part, and they are therefore called in-volun-ta-ry. 1. THE VOLUNTARY MUSCLES1 The Biceps Muscle. - When I place my left hand on the front surface of my right upper arm, and then draw up my W FIG. 88. Action of the Biceps Muscle. α= attachment of tendons to shoulder. P attachment of lower tendon to the radius. right forearm as far as possible, I feel the muscle in front of the humerus become shorter, thicker, and harder. By extending the forearm again, a tough cord or ten'don can be felt at the lower or distal end of the muscle. This tendon attaches the muscle to the radius bone. The proximal end of this muscle is covered by thick layers of flesh, but if these were removed, we should find two other tendons, which connect the muscle with projections on the shoulder blade (Fig. 88). We are now prepared for certain definitions. The muscle we have been studying is called the bi'ceps, from the fact that its upper 1 See "Laboratory Exercises," No. 33. = end has two heads or tendons (Latin bi= two + caput head). The central portion or the part that contracts is called the belly of the muscle. Since the biceps muscle is joined above to the shoulder blade and below to the radius, it therefore passes across two joints. When we lift a book with our forearm, the upper tendons remain practically unmoved; this end of the muscle is then called its origin. The tendon attached to the radius, however, is made to move considerably, and to this end is given the name insertion of the muscle. If, on the other hand, we climb a rope hand over hand, the elbow joint is held firm, and the motion takes place at the shoulder. Under these conditions the radius end is the origin, and the scapular end the insertion. By origin of a muscle is meant the end that moves least; by insertion, the end that moves most. In the majority of muscles one end is always origin, the other insertion. = = The Triceps Muscle. - If we straighten or extend the forearm as far as possible, the belly of a muscle behind the humerus is found to swell. This is the triceps muscle (Latin tri three+caput : head), so called because it has three tendons at its upper end. These tendons form the origin of the triceps, and are attached to the shoulder blade and to the humerus. The insertion of the muscle is on the projecting head of the ulna (commonly known as the "funny bone") (see Fig. 89). — Arrangement of Muscles in the Body. If the biceps muscle is made to contract, the forearm is brought upward or flexed. When the triceps exerts its force, the biceps relaxes and the forearm is straightened or extended. This illustrates the paired arrangement of muscles throughout the body; for a flexor muscle on one side of a joint is balanced by an extensor on the other side, which acts as its antagonist. Along the palm side of the forearm are the bellies of the flexor muscles that bend the fingers, while on the back of |