Myology

The Muscular System

The muscles (Musculi) are the active organs of motion. They are characterized by their property of contracting when stimulated. Muscular tissue is of three kinds: (a) Striated or striped; (b) non-striated, unstriped, or smooth; and (c) cardiac. Only the first of these varieties will be considered in this section. The striped muscles, being for the most part directly or indirectly connected with the skeleton, are often termed skeletal or somatic, while unstriped muscle may be spoken of as visceral or splanchnic. The former cover the greater part of the skeleton, and thus in a large measure determine the form of the animal. They are red in color, the shade varying in different muscles and under various conditions.

Muscles vary greatly in form, and may be classified as—(a) Long; (b) short; (c) flat; (d) ring-like or orbicular. Long muscles are found chiefly in the limbs, while the flat or broad muscles occur principally in the trunk, where they assist in forming the walls of the body cavities. The ring-like or orbicular muscles circumscribe orifices which they close, and are hence termed sphincters.

Attachments.—The muscles are attached to bones, cartilages, ligaments, fasciæ, or the skin. In all cases the attachment is by means of fibrous tissue, the muscle-fibers not coming into direct relation with the bone or cartilage. The perimysium of the muscle may fuse directly with the periosteum or perichondrium (fleshy attachment), or the union may be by means of intermediary fibrous structures called tendons or aponeuroses (tendinous attachment). Tendons may be funicular, ribbon-like, or in the form of membranous sheets; to the latter the term aponeurosis is commonly applied.

In certain positions, especially where tendons play over joints or are subjected to great pressure, sesamoid bones develop in the original tendon tissue. Some of these are large and constant, as the patella and the great sesamoids of the fetlock.

The accessory structures connected with the muscles are the synovial membranes and the fasciæ.

The synovial membranes are arranged in two principal forms: (a) Bursal; (b) vaginal. A bursa (Bursa mucosa) is a simple sac interposed between the tendon or muscle and some deeper seated structure—most commonly a bony prominence. A vagina tendinis or tendon sheath differs from a bursa in the fact that the synovial sac is folded around the tendon more or less completely, so that two layers can be distinguished; of these, the inner one adheres closely to the tendon, while the outer lines the groove or canal in which the tendon lies. The two layers are continuous along a fold termed the mesotendon. The arrangement is shown in Fig. . The articular synovial membranes in some places form extra-articular pouches, which facilitate the play of tendons.

The fasciæ are sheets of connective tissue, mainly of the white fibrous variety, with a greater or less admixture of elastic fibers in certain cases. Two layers may usually be recognized. Of these, the superficial fascia (Fascia subcutanea) is composed of loose connective tissue which may contain more or less fat and is subcutaneous. The deep fascia is composed of one or more layers of dense fibrous tissue spread over the surface of the muscles chiefly. Its deep face may be very loosely attached to the underlying structures or may fuse with the epimysium, tendons, bones, or ligaments. In some parts, especially the limbs, septal plates pass between the muscles and are attached to the bones or ligaments; these are termed intermuscular septa. In this way many muscles are inclosed in fibrous sheaths which hold them in position. Not uncommonly special bands stretch across the grooves in which tendons play, converting these into canals. Such bands are termed vaginal or annular ligaments. The deep fascia is often so distinctly tendinous in structure, furnishing attachment to special tensor muscles, as to render the distinction between fascia and aponeurosis quite arbitrary. Bursæ occur in certain situations between the fascia and underlying structures (subfascial bursæ), or between the fascia and the skin (subcutaneous bursæ).

It is convenient to divide the description of a muscle into seven heads, viz.: (1) Name, followed by important synonyms; (2) position and form; (3) attachments; (4) action; (5) structure; (6) relations; (7) blood and nerve supply.

1. The name is determined by various factors, viz.: (a) The action, e. g., extensor, adductor, etc.; (b) the shape, e. g., quadratus, triangularis; (c) the direction, e. g., rectus, obliquus; (d) the position, e. g., the subscapularis, iliacus; (e) the division (into heads, etc.), e. g., biceps, triceps, etc.; (f) the size, e. g., major, minor, etc.; (g) the attachments, e. g., sterno-cephalicus, mastoido-humeralis; (h) the structure, e. g., semitendinosus. In most cases two or more of these factors have combined to produce the name, e. g., adductor magnus, longus colli, obliquus externus abdominis.

2. The shape is, in many cases, sufficiently definite to allow the use of such terms as triangular, quadrilateral, fan-shaped, long, flat, fusiform, ring-like, etc.

3. The attachments are in most cases to bone, but many muscles are attached to cartilage, ligaments, fascia, the skin, etc. It is usual to apply the term origin to the attachment which always or more commonly remains fixed when the muscle contracts. The term insertion designates the movable attachment. Such a distinction cannot always be made, as the action may be reversible, or both attachments may be freely movable.

4. The action belongs rather to physiological study, but is briefly indicated in anatomical descriptions.

5. The structure includes the direction of the muscle-fibers, the arrangement of the tendons, the synovial membranes, and any other accessory structures, e. g., annular ligaments and reinforcing sheaths and bands. The relation of the muscle-fibers to the tendon varies, and this fact has given rise to special terms. Thus a muscle in which the fibers converge to either side of the tendon is termed bipennate; while one in which this arrangement exists only on one side of the tendon is called unipennate. The terms fleshy and tendinous are used to indicate the relative amounts of muscular and tendinous tissue. The muscular tissue is often spoken of as the belly (Venter) of the muscle. In the case of the long muscles, the origin is often termed the head (Caput). Muscles having two or more heads are called biceps, triceps, etc. Digastric muscles are those which have two bellies joined by an intermediate tendon. Ring-like muscles which circumscribe openings are termed sphincters, on account of their action.

6. The relations are, of course, important on surgical grounds.

7. The nerve-supply is of clinical interest, and is important for the determination of homologies.