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=.
[Illustration:
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.[41] 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.