The consideration of the many varieties of muscles, and the various modes in which they are attached to the bones that they are destined to move, belongs to the department of practical anatomy, and needs no mention here. As a general but by no means universal rule, a muscle has one attachment which is fixed, commonly spoken of as its origin, and a second, called its insertion, upon which it acts by approximating it to the origin. Muscles usually pass in a straight line between their two attachments, but sometimes they act round an angle by sliding over a pulley, or by means of a small bone in the tendon, like the patella.
The muscles are so attached that they are always slightly on the stretch, and thus, at the moment they begin to contract, they are in an advantageous position to bring their action to bear on the bones which they move. When the contraction ceases, the bones are drawn back to their former position without any sudden jerk or jar.
The muscles act upon the bones as levers, by working upon the short arm of the lever, so that more direct force is required on the part of a muscle than the weight of the body moved; but from this arrangement considerable advantages are gained, viz., that a small contraction of the muscle causes an extensive excursion of the part moved, and much greater rapidity of motion is attained.
All the three orders of levers are met with in the movements of the different bones of the skeleton; often, indeed, all three varieties are found in the same joint, as the elbow, where the simple extension and flexion motions of the biceps and triceps muscles give us good examples (Fig. 194).
The first order of lever is used when the triceps is the power and draws upon the olecranon, thus moving the hand and forearm around the trochlea, which acts as the fulcrum. This is shown in the upper diagram, in which the hand is striking a blow with a dagger.
The second order comes into play when the hand, resting on a point of support, acts as the fulcrum, and the triceps pulling on the olecranon is the power which raises the humerus, upon which is fixed the body or weight (middle diagram).
The third order may be exemplified by the action of the biceps in ordinary flexion of the elbow. Here the muscle, which is the power, is placed between the fulcrum - represented by the lower end of the humerus - and the weight which is carried by the hand (lower diagram).
The various groups of muscles which are so arranged as to assist each other when acting together, are called syner-getic, and those which, when contracting at the same time, oppose each other, are called antagonistic. The same muscles may, in different positions of a joint or in combination with other muscles, have totally different actions, at one time being synergetic and at another antagonistic. Thus, the sterno-mastoid muscle may, in different positions of the head, either bend the cranium backward or forward, and so cooperate with two sets of muscles which are definitely antagonistic to one another.