Opposing muscles. Compound muscular action.

308. Every muscle performing a motion has its opposing muscle or muscles, which perform the opposite motion. In the case of any two opposing muscles the one must be in some measure relaxed while the other is in action. Thus in alternately bending the elbow, and straightening it, there is alternate. action and relaxation in the two opposite muscles 4 and 7, as represented in Fig. 111. So in moving the head back and forth the muscles in front and rear are alternately contracted and relaxed. Paley very aptly compares this to the action of two sawyers in a pit, as they move the saw back and forth. The comparison, however, is not strictly true, because the relaxing muscle is never wholly relaxed. There is indeed in every muscle some amount of contraction which is independent of action through the nerves, whether it be reflex, or produced by the will. For this reason the muscles cut off in amputation of a limb retract. So also if the muscles on one side of the face be palsied, the muscles on the other side draw the mouth to that side. The mouth is held in the middle of the face by the equal action of pairs of muscles. The head, too, is held in equilibrium in the same way. In what is called wry-neck, this tonic contraction, as it is sometimes termed, is greater in the muscles on one side than it is on the other. In some cases a cure can be effected only by dividing the contracted muscles. In strabismus, or squinting, one of the straight muscles of the eyeball contracts. too strongly for its opposing muscle, and as in wry-neck, dividing the contracting muscle is often necessary to remedy the difficulty.

309. Most motions are not performed by single muscles, but by the joint and agreeing action of several, and sometimes many muscles. And as these muscles may vary to a great extent in their degree of contraction, the motions produced by them are not only compound, but are exceedingly varied. To illustrate this compound and varied character of motion, I will refer to a single example in which only two muscles are concerned in the motion. In Fig. 113 you see a pair of muscles, one of which is marked h, which extend from the large protuberances behind the ears to the top of the breast-bone. In the neck of a thin muscular person these muscles are very prominent. When they contract equally, the head is bent straight forward in the middle line between the muscles, and a line drawn from the middle of the forehead down to the breastbone would strike exactly at the point where these two muscles unite. But if one muscle contracts more strongly than the

Variety in muscular action. Exemplified in the tongue.

other, the head as it bows forward bows towards the side on which is the strongest contraction. And as the degrees of contraction in these two muscles may be endlessly varied, so there may be an endless variation in the degree of inclination of the head to one side or the other, as it is bent forward. If then so great a variety in the direction of motion may be produced by variation in the degrees of action in two muscles, you can readily see that an almost infinite variety of motion must result from this variation, where many muscles are called into action. 310. I know not any part of the body, which exemplifies in so palpable a manner the compound and diversified character of muscular motion as the tongue. It is mostly a bundle of muscular fibres, apparently mingled together in confusion, but really arranged in perfect order, so that it can be moved with great definiteness in all directions, forward, backward, upward, downward, to either side, and in all intermediate directions. If you stand before a glass, and opening your mouth, move the tongue rapidly about in all these directions, you think of a harlequin performing his antics. But all this wonderful variety of movement is produced in obedience to the definite action of nerves, whose fibres are mingled with the muscular fibres of the tongue. And in order to produce each motion there is an agreement of action not between merely many of these fibres, but between multitudes of them.

311. With the view which I have given you of the compound and varied character of muscular motion, you are prepared to take a general survey of the muscular system. For this purpose I call your attention to a side view of the muscles of the body as presented in Fig. 122. I must premise, that you can get no idea from this Figure of the number of the muscles in the body, for you see here only the outer layer of muscles, and there are many muscles concealed by them. You observe that they are of various shapes and sizes, according to the motions which they are designed to produce, and the circumstances in which they are placed. They are round, long, short, flat, fan-shaped, circular, serrated, &c. I will point out some of them. At a is the very large muscle that makes the fleshy prominence at the upper part of the arm, and the office of which is to raise the arm, carrying it out from the body. You observe that its fibres are not all arranged alike but lie in different directions. The result is, that while the arm is raised by the muscle as a whole, it may be carried at the same time forward or backward by the varying action of

General description of the muscles in various parts.

these different fibres. There are many of the muscles of the body which are made thus to produce various results by variation of the action of different parts of the same muscle. And the regulation of this variation by the nerves is one of the most wonderful and mysterious things which we find in our study of the nervous system. For each fibre in the cases referred to is told, as we may express it, just how much it must do in order to produce the requisite general motion of the muscle. It is manifestly much more wonderful thus to produce various but accurately graduated contraction in different parts of the muscle, than to produce an uniform contraction in all its fibres.

At

312. I go on with my notice of the particular muscles. At b is the biceps muscle, which bends the forearm upon the arm, and at c is another muscle that assists the biceps. At e is the large muscle in the back of the arm, which acts in opposition to the biceps, and straightens the forearm upon the arm. d is a muscle which rolls the radius outwards, and thus turns the palm of the hand upward as seen in the Figure. At g is a very large broad muscle coming from the whole length of the back, and at the axilla or arm-pit, its fibres are collected, twisted, and folded upon each other. The muscle is fastened by a stout tendon to the upper and back part of the bone of the arm, and its office is to pull the arm backwards and downwards. At h is a serrated muscle, which rising from the ribs, goes to the shoulder-blade, and serves to draw the shoulderblade forwards. At i is one of the broad muscles of the abdomen. At and k are two large muscles that move the thigh. At o and p, as seen on the right thigh, and at n, as seen on the left, are three large muscles, which are fastened to the kneepan, and serve to throw the leg forward as described in § 304. At q is the tendon that forms the outer hamstring, and at r are the two tendons which form the inner one. The muscles to which these tendons belong, serve to bend the leg upon the thigh, drawing it upward and backward. At s is the muscle which makes the bulk of the calf of the leg. It lifts the heel upward and backward, and it is seen in action in the right leg of the Figure. Its strong tendon which is attached to the top of the heel bone is called, on account of its strength, the tendon of Achilles. This muscle is in Fig. 109, the power P which raises the weight of the body, W, on the fulcrum, F, as the heel is raised from the ground in walking.

313. In Fig. 123 you have a rear view of the muscles. At