It is hardly necessary to notice the relation which bones and muscles have to movement, and the most unlearned in anatomy need only be reminded that muscles are adherent to certain parts of bones. The bone, by itself, is motionless, and the force which can move it, and with it, the surrounding flesh and skin, acts through the muscles, and these consist of vast numbers of long microscopic fibrils, placed side by side, and adherent, at both ends, to different bones. The fibrils have a vast amount of energy in them, and they can contract, or, in other words, shorten; the diminution in length being accompanied by a display of force. As the fibrils shorten, they tend to bring the motionless bones closer together, and to impart motion, which may be rapid, and more or less forcible. If one bone is stationary, the other may be brought towards it by the muscular contraction, or if both are not fixed, both may move. The nervous force produces the muscular contraction, whose vigour and lasting power depend a great deal upon the supply of blood sent to the fibrils through the blood vessels (arteries), and removed through the veins.
BONES OF THE FORE-ARM AND ARM OF THE GORILLA—SIDE VIEW.
SHOULDER OR BLADE-BONE.
In the principal act of climbing hand-over-hand, a bough or some stationary object is grasped by the fingers, the arm being straight, and the body hanging, as it were, to it. The first motion is the lifting up of the arm; the second is the grasping with the hand; and the third is the bending of the straight elbow, and bringing the shoulder up nearer the fixed point, or the part grasped. Whilst this is being done the body is not limp, but more or less stiffened by the spine, which runs down the back, and consists of many bones, being made rigid by the contraction of many small muscles. Now the bones and muscles of all the parts of the body engaged in climbing are so arranged that the spine shall not suffer any jarring, but shall be lifted up safely. Were all the muscles which pull upon the arms attached to it, every unusual effort would drag it almost to pieces, so there is a wide flat bone placed between the spine and the arm. This so-called blade-bone is jointed by a ball and socket joint to the arm-bone, but is only united to the spine and back part of the head by muscles. Muscles start from the spine to the blade-bone, from the blade-bone to the bones of the arm and fore-arm, and from these last to the bones of the fingers, and by their shortening or contraction, the fingers being stationary, the body is at last brought closer to them.
In order to explain the first motions of climbing, it is necessary to remark that on looking at the skeleton of the Gorilla the shoulder-blades are seen to be of the same general shape as those of man; they are much larger, however, and there are some anatomical points about them, which clearly have to do with the ability of the great Ape to keep its arms up for a long time, and to pull up its heavy body when the hands and fore-arms are fixed and immovable by clasping. One muscle, which in ourselves forms the cushion on the shoulder, and reaches down the outside of the arm for a little distance, is called the deltoid or Δ-shaped muscle, and its especial duty is, when the shoulder-blade is fixed, to lift up the arm by its contraction. The movement is permitted because between the spots where the muscles are adherent to the blade-bone on the one hand, and to the outside of the arm-bone on the other, a distance of several inches, there is a joint like a ball and socket. The muscle is not attached to a flat surface on the blade-bone, but to a raised edge, which runs rather obliquely, and is called the spine of the bone. Now this muscle is of immense importance to the Gorilla, as may be imagined from the nature of its function or office: it is placed in the same position as in man, and between the same kind of bones, but the spine of the blade-bone is longer, broader, and more slantingly set in the Ape, so that extra strength and greater power are attained.
This spine, or rather raised ridge, can be felt when we place the right hand over the left shoulder as far as possible, keeping the fingers between the neck and the end of the shoulder, and its slanting position can be traced best in the Gorilla; and it may be mentioned, that in the Chimpanzee the direction is much more oblique. Above this spine of the blade-bone there is the upper part of the blade, and it is covered with muscle, the space thus occupied being much larger in the Gorilla than in man. This muscle starts from this bone, to which it is attached, and is united to the arm-bone, close to its joint with the blade-bone; it is larger in the Gorilla than in us, and one of its uses is to assist the deltoid just mentioned.
There is rather an interesting arrangement in the old Gorillas, which is not found in the young or in man, and which appears to have to do with the power of this muscle and its prolonged action. The muscle is well supplied with blood, and the nerve which endows it with energy is particularly well prevented from being compressed during the movements of the muscles amongst which it runs, any compression being very injurious. The upper edge of the blade-bone is notched, and a dense tissue or ligament stretches from one point of the notch to the opposite one, enclosing a small open space; now the nerve runs through this space, and is protected by the hard tissues of bone and ligament from the contraction of the soft muscles. In the old Gorilla a further protection is found in the presence of a little projection of bone in this space, which acts as a greater preventer of pressure.
After passing through this space the nerve enters the very substance of the muscle, and is distributed to its fibrils.