Part 55

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=.

FIG. 171.—DIAGRAMS OF CROSS-SECTIONS OF TENDON SHEATH (A) AND BURSA (B); T, TENDON. ]

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. 171. 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.

FASCIÆ AND MUSCLES OF THE HORSE

PANNICULUS CARNOSUS

The =panniculus carnosus= (Musculus cutaneus) is a thin muscular layer developed in the superficial fascia. It is intimately adherent in great part to the skin, but has very little attachment to the skeleton. It does not cover the entire body, and may be conveniently divided into facial, cervical, thoracic, and abdominal portions, each of which will be described with the muscles of the corresponding region.

THE FASCIÆ AND MUSCLES OF THE HEAD

The muscles of the head may be divided into three groups, viz.: (1) Superficial muscles, including the panniculus and those of the lips, cheeks, nostrils, eyelids, and external ear; (2) the orbital muscles; (3) the muscles of mastication.

The =superficial fascia= forms an almost continuous layer, but is very scanty around the natural orifices. It contains a number of the thin superficial muscles, so that care must be exercised in removing the skin. Over the frontal and nasal bones the fascia blends with the periosteum.

The =deep fascia= is of special interest in three regions. The =temporal fascia= covers the temporalis muscle, and is attached to the parietal and frontal crests internally, and to the zygomatic arch externally. The =buccal fascia= covers the buccinator muscle and the free part of the outer surface of the ramus of the jaw. Superiorly it is attached to the facial crest, and posteriorly it forms a band (Ligamentum pterygomandibulare) which stretches from the hamulus of the pterygoid bone to the mandible behind the last molar tooth. It is directly continuous with the =pharyngeal fascia=, which is attached to the great and thyroid cornua of the hyoid bone, covers the lateral walls of the pharynx, and blends dorsally with the median raphé of the constrictor muscles of the latter.

SUPERFICIAL MUSCLES

1. =Panniculus carnosus.=—The facial panniculus (M. cutaneus faciei) consists of a thin and usually incomplete muscular stratum, which covers the submaxillary space and the masseter muscle. A branch from it passes forward to the angle of the mouth and blends with the orbicularis oris; this part (M. cutaneus labiorum) retracts the angle of the mouth. (A number of the superficial muscles of the face may be considered modified parts of the panniculus, e. g., the corrugator supercilii, malaris, zygomaticus, etc.)

MUSCLES OF THE LIPS AND CHEEKS

1. =Orbicularis oris.=—This is the sphincter muscle of the mouth; it is continuous with the other muscles which converge to the lips. It lies between the skin and the mucous membrane of the lips, and is intimately adherent to the former. Most of the fibers run parallel to the free edges of the lips and have no direct attachment to the skeleton.

Action.—It closes the lips.

Blood-supply.—Palato-labial, facial, and mental arteries.