Part 25

The suspensory or superior sesamoidean ligament is more distinctly muscular than in the horse—indeed in the young animal it consists almost entirely of muscular tissue. At the distal third of the metacarpus it divides into three branches. These give rise to five subdivisions, either by bifurcation of the lateral branches or trifurcation of the middle branch. The four lateral bands end on the sesamoid bones and the distal end of the large metacarpal bone, and detach slips to the extensor tendons. The middle band passes through the groove between the two divisions of the distal end of the metacarpus, and divides into two branches which join the tendons of the proper extensors of the digits; it sends fibers also to the interdigital lateral ligaments and to the central sesamoids. About the middle of the metacarpus the suspensory ligament detaches a band which unites lower down with the superficial flexor tendon, thus inclosing the tendon of the deep flexor of the digit; it also blends with the thick fascia of the region. The latter gives off a band on either side to the accessory digits, and a tendinous band descends from each accessory digit to the third phalanx and sesamoid bone, blending with the tendon of the corresponding proper extensor.

=Pig.=—There are four metacarpo-phalangeal joints, each of which has a capsule, lateral, intersesamoidean, and crucial sesamoidean ligaments. Since distinct interosseous muscles are present, there are, of course, no suspensory ligaments.

=Dog.=—There are five metacarpo-phalangeal joints, each having its own capsule and indistinct lateral ligaments. A small sesamoid bone occurs in the anterior part of each capsule, over which the corresponding extensor tendon plays. The intersesamoidean ligaments do not extend above the sesamoids. The cruciate ligaments are present, as well as a fibrous layer which attaches the distal margins of the sesamoids to the posterior surface of the proximal end of the first phalanx.

INTERPHALANGEAL JOINTS

=Ox.=—The two proximal joints have separate capsules, and broad, but rather indistinct, lateral ligaments. Each joint has also two central and two lateral volar ligaments. The central ligaments are largely fused to form a strong band. The lateral ones extend from the borders of the first phalanx to the proximal end of the second phalanx.

The distal interphalangeal joints have, in addition to the capsules and lateral ligaments, bands which reinforce them on either side. The central or interdigital pair arise in the depressions on the distal ends of the first phalanges, receive fibers from the second phalanges, and end on the interdigital surfaces of the third phalanges at the margin of the articular surface. The lateral pair have a similar course, but are thinner, and end on the corresponding third sesamoid. An elastic band crosses the front of the second phalanx obliquely, from the distal end of the first phalanx to the extensor process of the third phalanx.

The =crucial= or =inferior interdigital ligaments= (Ligamenta cruciata interdigitalia) are two strong bands which limit the separation of the digits. They are attached above to the lateral (or abaxial) eminences on the proximal ends of the second phalanges (blending with the lateral ligaments), cross the deep flexor tendon obliquely, and reach the interdigital space, where they intercross and blend. Most of the fibers end on the third sesamoid of the opposite side, but some are attached to the interdigital aspect of the second phalanx and the third sesamoid of the same side. In the sheep there is, instead of the foregoing, a transverse ligament which is attached on either side to the interdigital surfaces of the second and third phalanges and the third sesamoid bone. It is related below to the skin, above to a pad of fat.

=Pig.=—The interphalangeal joints of the chief digits resemble in general those of the ox. The inferior interdigital ligament resembles, however, that of the sheep, and is intimately adherent to the skin. There is, besides, a remarkable arrangement of ligaments which connect the small digits with each other and with the chief digits.

This apparatus is somewhat complex, but its chief features are as follows: A superior interdigital ligament is attached on either side to the third phalanges of the small digits, while centrally it blends with the annular ligaments of the flexor tendons behind the metacarpo-phalangeal joints of the chief digits. Two bands (central longitudinal interdigital ligaments) arise on the bases of the small digits, cross the flexor tendons obliquely downward and inward, pass through the superior interdigital ligament, and blend below with the inferior interdigital ligament. Two lateral bands (lateral longitudinal interdigital ligaments) are attached in common with the superior interdigital ligaments to the third phalanges of the small digits, and blend below with the outer part of the inferior interdigital ligament.

=Dog.=—Each joint has a capsule and two lateral ligaments. The distal joints have also two elastic =dorsal ligaments= (Ligamenta dorsalia), which extend from the proximal end of the second phalanx to the ridge at the base of the third phalanx (Fig. 133). They produce dorsal flexion of the joint, and thus raise or retract the claws when the flexor muscles relax. The distal sesamoids are represented by complementary cartilages attached to the volar margins of the articular surfaces of the third phalanges.

Three interdigital ligaments restrict the spreading apart of the digits. Two of these cross the volar surface of the proximal parts of the chief digits, _i. e._, one for the second and third, the other for the fourth and fifth; they blend with the annular ligaments on either side. The third ligament is attached on either side to the foregoing ligaments and the annular ligaments of the third and fourth digits, and curves downward centrally, ending in the large pad on the paw.

ARTICULATIONS OF THE PELVIC LIMB

SACRO-ILIAC JOINT

This joint and the pelvic ligaments present no very striking differences in the other animals. The sacro-sciatic ligament in the dog is a narrow but strong band which extends from the posterior part of the lateral margin of the sacrum to the tuber ischii; it is the homologue of the ligamentum sacro-tuberosum of man.

HIP JOINT

=Ox.=—The shallowness of the acetabulum is compensated by the greater size of the marginal cartilage. The head of the femur has a smaller radius of curvature than that of the horse, and the articular surface extends a considerable distance outward on the upper surface of the neck. The round ligament is entirely intraarticular; it is small, and sometimes absent. The pubo-femoral or accessory ligament is absent.

There are no important differences in the other animals.

STIFLE JOINT

=Ox.=—There is a considerable communication between the femoro-patellar and inner femoro-tibial joint cavities; this is situated as in the horse, but is wider. A small communication with the external femoro-tibial capsule sometimes occurs. The two femoro-tibial capsules usually communicate. The middle patellar ligament is not sunken, as there is no groove on the tuberosity of the tibia where it is attached. The external patellar ligament fuses completely with the tendon of insertion of the biceps femoris, and a synovial bursa is interposed between them and the external condyle of the femur.

In the other animals there is a single ligamentum patellæ, and the synovial sacs communicate so freely as to constitute a common joint cavity. In the dog the semilunar cartilages are united anteriorly by a transverse ligament, and the posterior part of the capsule contains the two Vesalian sesamoids, which articulate with the condyles of the femur.

TIBIO-FIBULAR JOINTS

=Ox.=—The proximal end of the fibula fuses with the external condyle of the tibia. The distal end remains separate, and forms an arthrosis with the distal end of the tibia; the movement here is imperceptible, as the two bones are closely united by strong peripheral fibers.

=Pig.=—The superior joint is provided with a capsule which is reinforced in front and behind by fibrous tissue.

The interosseous ligament attaches the shaft of the fibula to the outer border of the tibia.

The inferior joint is included in the capsule of the hock joint, and is strengthened by oblique anterior and posterior ligaments. There is also an interosseus ligament.

=Dog.=—The arrangement is essentially the same as in the pig, but there is no interosseous ligament in the inferior joint.

HOCK JOINT

=Ox.=—There is very considerable mobility at the first intertarsal joint, the capsule of which is correspondingly roomy. The short external lateral ligament is attached below on the tibial tarsal only. A strong transverse ligament attaches the external malleolus (distal end of the fibula) to the back of the tibial tarsal bone. The dorsal or oblique ligament is narrow and thin.

=Pig.=—The arrangement in general resembles that of the ox.

=Dog.=—The long lateral ligaments are very small, and the short ones double. The plantar ligament is weak, and ends on the fourth metacarpal bone. No distinct dorsal (or oblique) ligament is present. Movement occurs almost exclusively at the tibio-tarsal joint.

The remaining joints resemble those of the thoracic limb.

THE MUSCULAR SYSTEM

MYOLOGY

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. 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.

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.

_Nerve-supply._—Facial nerve.

2. =Levator nasolabialis= (Levator labii superioris alæque nasi).—This thin muscle lies directly under the skin, and chiefly on the lateral surface of the nasal region.

_Origin._—The frontal and nasal bones.

_Insertion._—(1) The upper lip and the outer wing of the nostril; (2) the commissure of the lips.

_Action._—(1) To elevate the upper lip and the commissure; (2) to dilate the nostril.

_Structure._—The muscle arises by a thin aponeurosis. The belly is also thin, and divides into two branches, between which the lateral dilator of the nostril passes. The dorsal branch reaches the nostril and upper lip, blending with the lateral dilator; the ventral one is much smaller, and blends at the labial commissure with the orbicularis and buccinator.

_Relations._—Superficially, the skin, fascia, and lateral dilator (in part); deeply, the levator labii superioris proprius, lateral dilator (in part), buccinator, branches of the facial vessels and nerve, and the infraorbital artery and nerve.

_Blood-supply._—Facial and palato-labial arteries.

_Nerve-supply._—Facial nerve.

3. =Levator labii superioris proprius.=—This lies on the dorso-lateral aspect of the face, partly covered by the preceding muscle.

_Origin._—The lacrimal, malar, and maxillary bones at their junction.

_Insertion._—The upper lip, by a common tendon, with its fellow.

_Action._—Acting with its fellow, to elevate the upper lip. This action, if carried to the fullest extent, results in eversion. In unilateral action the lip is drawn upward and to the side of the muscle acting.

_Structure._—The muscle has a short, thin tendon of origin. The belly is at first flattened, but becomes narrower and thicker, then tapers over the false nostril, to terminate in a tendon. The tendons of the two muscles unite over the alar cartilages of the nostrils, forming an expansion which spreads out in the substance of the upper lip.

_Relations._—Superficially, the skin, the levator nasolabialis, and the angular vessels of the eye; deeply, the superior and transverse dilators of the nostril and the infraorbital artery and nerve.

_Blood-supply._—Facial artery.

_Nerve-supply._—Facial nerve.

4. =Zygomaticus.=—This very thin muscle lies immediately under the skin of the cheek.

_Origin._—The fascia covering the masseter muscle below the facial crest.

_Insertion._—The commissure of the lips, blending with the buccinator.

_Action._—To retract and raise the angle of the mouth.