There are three varieties of muscular tissue in the human body:
- striped or voluntary.
- unstriped or involuntary
All the muscles described in this section, together with those of the eyeball, ear; tongue, palate; larynx and pharynx, which are considered with the anatomy of these organs, consist of striped or voluntary muscle.
The voluntary muscles are connected with the bones, cartilages, ligaments, and skin, either directly, or through the intervention of fibrous structures, called tendons or aponeuroses. Where a muscle is attached to bone or cartilage, the fibers end in blunt extremities upon the periosteum or perichondrium, and do not come into direct relation with the osseous or cartilaginous tissue. Where muscles are connected with the skin, they lie as a, flattened layer beneath it, and are united with its areolar tissue by larger or smaller bundles of fibers. A muscle gains attachment at both of its extremities, and this arrangement enables the power of contractility which it possesses to effect a movement of some part or parts of the body.
When a muscle contracts, one of its attachments remains relatively stationary while the other is approximated to it. The term origin is used to designate the more fixed attachment and the term insertion to designate the movable point at which the force of the muscle is applied. As a general rule, so far as the limbs are concerned, the origin is the more proximal and the insertion the more distal extremity of a muscle. In accordance with these definitions the contraction of a muscle results in the approximation of its insertion to its origin, but the terms are arbitrary and used for convenience only, and it frequently happens that the contraction of a muscle may result in the approximation of its origin to its insertion. The Gluteus maximus provides an illustration of this fact. When it assists in extending the flexed thigh, its insertion into the gluteal tuberosity of the femur is then approximated to its origin from the dorsal aspect of the sacrum. On the other hand, when the body is bent forwards at the hips, the Glutei maximi play the principal part in restoring it to the erect posture and, in this movement, the origin is approximated to the insertion. In many cases, however, the character of the insertion is such that it can never become the fixed point. For example, the contraction of the muscles of facial expression, which take origin from bone and are inserted into skin, can only result in movements of the skin, i.e. in the approximation of the insertion to the origin.
The form of a, muscle depends on the number and arrangement of its constituent fibers. When the fibers are arranged parallel or nearly parallel to what may be termed the ‘line of pull’ of the muscle, the full advantage of the muscular contractility is available and the maximum range is obtained. Such muscles may be quadrilateral, like the Thyrohyoid; fusiform, life the Flexor carpi radialis; or straplike, such as the Sartorius. If, on the other hand, the fibers are arranged obliquely in relation to the ‘ line of pull,’ the range is diminished, for the force of the muscular action can then be resolved into two components, one acting in the `line of pull ‘ and the other at right angles to it and therefore valueless so far as the range of the muscle is concerned. This oblique arrangement of the fibers is seen (a) in unipennate muscles such as the Flexor pollicis longus, in which the tendon of insertion extends upwards along one border of the muscle, (b) in bipennate muscles, such as the Rectus femoris, where the tendon extends upwards through the middle of the muscle, and (c) in multipennate muscles, such as the Deltoid, where a number of extensions pass upwards into the muscle from its tendon of insertion. The same arrangement is seen (d) in trianqular muscles, such as the Temporalis, in which the muscular fibers converge on an apical tendon. As a rule the various forms of pennate and triangular muscles contain a larger number of fibers than fusiform, quadrilateral or straplike muscles, and they are therefore found in situations where increased power is essential.
The character of a muscular attachment is subject to considerable variation, but in every case the actual attachment is effected through the medium of white connective tissue fibers. These fibers way be collected together, forming either thin, expanded sheet-like aponeuroses or cord-like tendons; or they may be broken up so that the attachment appears to be party- by fleshy and partly by tendinous fibers; or they may be so widely separated that their presence cannot be appreciated by the naked eye and the attachment is apparently effected by means of fleshy fibers. Owing to its active contractility muscle tissue has a higher metabolic rate than white fibrous tissue. Therefore the substitution of tendon for muscle, in cases where the arrangement does not involve any diminution in range, is metabolically economical. During contraction a muscle fiber may shorten by thirty to forty percent of its uncontracted length. As all the fibers in a given muscle are attached by one end to the tendon of origin, or its prolongation into the muscle and by the other to the tendon of insertion the maximum amount of shortening of the muscle is equal to the maximum amount of shortening of any one fiber, provided that all the fibers in the muscle are uniform in length, e.g. Sartorius. Owing to the rigidity of the bones, the amount of shortening possible is only a small percentage of the distance between the origin and the insertion, and the length of muscle fibers is adapted to their requirements. When, therefore, the muscle fibers are arranged parallel to the ‘line of pull,’ the muscular belly may be relatively short, and it is then provided with a long tendon-an arrangement which is both economical and efficient.
Attachments of muscles exert a definite influence in the modeling of the bones of the skeleton and, when the attachment is tendinous in character, localized, elevated areas are usually present. On the other hand, when the muscle is attached by fleshy fibers the bony surface is smooth, and shows no corresponding elevations.
The muscles of the limbs with very few exceptions are inserted immediately distal to the joint on which they exert their principal action, This arrangement implies a loss of mechanical advantage; but what is lost in power is gained in the speed with which the hand or the foot, as the case may be, is moved over a wide range. For example, when the Brachialis muscle contracts, its insertion into the coronoid process of the ulna moves round an area of a circle with a very short radius, but in the same period of time the hand moves round the corresponding are of a circle whose radius is the length of the forearm and hand.
Theoretically a muscle is capable of acting on every joint over which it passes, and the particular movements in which it takes part depend on the relationship of its `line of pull’ to the axes of the movements of the joint.
For example, a muscle which passes in front of the transverse axis of the shoulder-joint will take part in the movement of flexion and, if it passes below the joint, it will also take part in adduction. It is movements, however; and not individual muscles which are represented in the cerebral cortex and it does not follow that a muscle will participate in a given movement because the mechanics of its attachments enable it to do so.
When a movement is carried out, a definite combination of muscles is called into play, and no muscle can be omitted nor can one be added, voluntarily. One muscle or more of the combination is the prime mover, and its active contraction necessarily involves the relaxation of its antagonist. The full effect, however, of the contraction of a prime mover can be obtained only when one of its attachments (usually the origin) is fixed; therefore, in the case of limb movements, every contraction of the prime movers is accompanied by the contraction of groups of fixation muscles. For example, the Deltoid is the prime mover in abduction of the arm and it can exert its full effect only when its clavicular and scapular origins are fixed. As a result abduction of the arm is always accompanied by contraction of muscles inserted into the scapula; and they are the fixation muscles for that movement. Some muscles pass over several joints, although they are prime movers of one joint only; and it may be necessary to counteract the effect of their action on the intermediate joints in the interests of efficiency. For this purpose a group of synergic muscles is brought into play. When the fist is clenched firmly, the prime movers are the flexors of the fingers, the flexors and adductor of the thumb and the Opponens pollicis muscle. The long flexors pass in front of the wrist joint and are able to flex that joint; but only at the expense of the power with which they act on the digits. The extensors of the wrist, which are the synergist muscles for the movement, are therefore thrown into contraction in order to steady the wrist and obviate this loss of power. It should be observed, however, that there is no essential difference between fixation and synergic muscles, in so far as they both prevent waste of power and loss of efficiency on the part of the prime movers.
Under certain conditions the muscular groups which are called into play for some movements are profoundly altered by the action of gravity. When the body is in the erect posture, flexion of the trunk is initiated by the ventral muscles, but thereafter the movement is carried out by the action of gravity, controlled and regulated by the Sacrospinales. The abducted arm is brought down to the side in a similar manner. In this instance the action of gravity is controlled by the Deltoid muscle, and the adductor muscles do not participate in the movement unless resistance is encountered.
It will now be apparent that the efficient performance of any movement depends on the proper co-ordination of prime movers, antagonists and fixation and synergic muscles, and this co-ordination is ensured by the manifold connections which exist within the central nervous system.