Part 24
=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=.
[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.
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.
[Illustration:
FIG. 172.—MUSCLES OF HEAD OF HORSE, LATERAL VIEW. THE PANNICULUS IS REMOVED.
_a_, Levator labii superioris proprius; _b_, levator nasolabialis; _c_, mastoido-humeralis; _d_, sterno-cephalicus; _d′_, tendon of _d_; _e_, omo-hyoideus; _f_, dilatator naris lateralis; _g_, zygomaticus; _h_, buccinator; _i_, depressor labii inferioris; _k_, orbicularis oris; _l_, dilatator naris superior; _m_, masseter; _n_, parotido-auricularis; _o_, zygomatico-auricularis; _p_, interscutularis; _p′_, fronto-scutularis, pars temporalis; _q_, cervico-auricularis profundus major; _r_, cervico-auricularis superficialis; _s_, obliquus capitis anterior; _t_, splenius; v, stylo-maxillaris; _y_, mastoid tendon of mastoido-humeralis; _2_, posterior, _3_, anterior, border of external ear; _8_, scutiform cartilage; _9_, zygomatic arch; _10_, depression behind supraorbital process; _18_, temporo-mandibular articulation; _27_, facial crest; _30′_, angle of jaw; _37_, external maxillary vein; _38_, jugular vein; 39, facial vein; _40_, parotid duct; _41_, transverse facial vein; _42_, masseteric vein; _43_, facial nerve; _44_, parotid gland; _45_, chin; _x_, wing of atlas. (After Ellenberger-Baum, Anat. für Künstler.) ]
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.
[Illustration:
FIG. 173.—MUSCLES OF HEAD OF HORSE, DORSAL VIEW. THE PANNICULUS IS REMOVED.
_a_, Levator labii superioris proprius; _a′_, common tendon of a with opposite muscle; _b_, levator nasolabialis; _f_, dilatator naris lateralis; _g_, zygomaticus; _l_, dilatator naris superior; _n_, parotido-auricularis; _o″_, scutulo-auricularis superficialis superior; _p_, interscutularis; _p′_, fronto-scutularis, pars temporalis; _r_, cervico-auricularis superficialis; _u_, corrugator supercilii; _x_, transversus nasi; _2_, posterior, _3_, anterior, border of external ear; _8_, scutiform cartilage; _9_, zygomatic arch; _10_, supraorbital depression; _35_, inner wing of nostril, containing lamina of alar cartilage; _39_, facial vein. (After Ellenberger-Baum, Anat. für Künstler.) ]
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.
_Structure._—Fleshy, with a thin aponeurotic origin.
_Relations._—Superficially, the skin; deeply, the buccinator.
_Blood-supply._—Facial artery.
_Nerve-supply._—Facial nerve.
5. =Incisivus superior= (Depressor labii superioris).—This lies under the mucous membrane of the upper lip.
_Origin._—The alveolar border of the premaxilla from the second incisor to the first cheek tooth.
_Insertion._—The upper lip.
_Action._—To depress the upper lip.
6. =Incisivus inferior.=—This is arranged in the lower lip like the preceding muscle in the upper one.
_Origin._—The alveolar border of the mandible from the second incisor to a point near the first cheek tooth.
_Insertion._—The skin of the lower lip and the prominence of the chin.
_Action._—To raise the lower lip.
7. =Mentalis= (Levator menti).—This is situated in the prominence of the chin. Its fibers arise from each side of the body of the mandible and are inserted into the skin of the chin. It is mingled with fat and strands of connective tissue, in which the roots of the tactile hairs are embedded. It raises and corrugates the skin to which it is attached.
8. =Depressor labii inferioris.=—This muscle lies on the outer surface of the ramus of the mandible, along the ventral border of the buccinator.
_Origin._—The alveolar border of the mandible near the coronoid process and the maxillary tuberosity, in common with the buccinator.
_Insertion._—The lower lip.
_Action._—To depress and retract the lower lip.
_Structure._—The tendon of origin and the belly are fused with the buccinator as far forward as the first cheek tooth. From this point forward the belly is distinct and rounded, terminating in a tendon which spreads out in the lower lip, blending with the orbicularis and the muscle of the opposite side.
_Relations._—Superficially, the skin, masseter, facial vessels, and parotid duct; deeply, the mandible and inferior labial artery.
_Blood-supply._—Facial artery.
_Nerve-supply._—Facial nerve.
9. =Buccinator.=—This muscle lies in the lateral wall of the mouth, extending from the angle of the mouth to the maxillary tuberosity.
_Origin._—The external surface of the maxilla above the interdental space and the molar teeth; the alveolar border of the mandible at the interdental space and also posteriorly where it turns upward to the coronoid process; the pterygo-mandibular ligament.
_Insertion._—The angle of the mouth, blending with the orbicularis oris.
_Action._—To flatten the cheeks, thus pressing the food between the teeth; also to retract the angle of the mouth.
_Structure._—Two layers may be recognized. The =superficial layer= (Pars buccalis) extends from the angle of the mouth to the masseter. It is incompletely pennate, having a longitudinal raphé on which most of the muscle-fibers converge. The upper fibers are directed chiefly downward and backward, the lower ones upward and backward. The =deep layer= (Pars molaris) consists mainly of longitudinal fibers. It blends in part with the superficial layer of the orbicularis; it has a small tendinous attachment to the coronoid process behind, and is united below with the depressor labii inferioris.
_Relations._—Superficially, the skin and fascia, the zygomaticus, levator nasolabialis, lateral dilator of the nostril, the superior buccal glands, the parotid duct, the facial vessels, and branches of the facial nerve; deeply, the mucous membrane of the mouth and the inferior buccal glands.
_Blood-supply._—Facial and buccinator arteries.
_Nerve-supply._—Facial nerve.
MUSCLES OF THE NOSTRILS
1. =Levator nasolabialis.=—This has been described (p. 214).
2. =Dilatator naris lateralis= (M. caninus).—This thin, triangular muscle lies on the lateral nasal region, and passes between the two branches of the levator nasolabialis.
_Origin._—The maxilla, close to the anterior extremity of the facial crest.
_Insertion._—The outer wing of the nostril.
[Illustration:
FIG. 174.—NASAL AND SUPERIOR LABIAL MUSCLES OF HORSE.
_a_, _a′_, Dilatator naris transversus; _b_, levator labii superioris proprius; _b′_, tendon of _b_; _b″_, common tendon of two levatores labii superioris proprii; _c_, _c′_, dilatator naris inferior; _d_, _e_, dilatator naris superior; _f_, orbicularis oris; _g_, levator nasolabialis, a portion of which is removed; _h_, dilatator naris lateralis (cut); _i_, cornu of alar cartilage; _k_, nostril; _k′_, false nostril; _l_, nasal diverticulum; _m_, nasal bone. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ]
_Action._—To dilate the nostril.
_Structure._—The muscle has a flat tendon of origin, passes between the two branches of the levator nasolabialis, and spreads out in the external wing of the nostril. The lower fibers blend with the orbicularis oris.
_Relations._—Superficially, the skin, fascia, and the labial branch of the levator nasolabialis; deeply, the maxilla and the nasal branch of the levator nasolabialis.
_Blood-supply._—Facial artery.
_Nerve-supply._—Facial nerve.
3. =Dilatator naris transversus= (M. transversus nasi).—This is an unpaired, quadrilateral muscle, which lies between the nostrils. It consists of two layers.
_Attachments._—Superficial layer, the superficial faces of the laminæ of the alar cartilages; deep layer, the convex edges of the cornua of the same.
_Action._—To dilate the nostrils.
_Structure._—It is composed of transverse fleshy fibers, which blend below with the orbicularis.
_Relations._—Superficially, the skin, fascia, and tendinous expansion of the levator labii superioris proprius; deeply, the alar cartilages, the extremity of the septum nasi, and the palato-labial artery.
_Blood-supply._—Palato-labial artery.
_Nerve-supply._—Facial nerve.
4. =Dilatator naris superior= (Pars dorsalis m. lateralis nasi).—This very thin muscle occupies the angle between the nasal process of the premaxilla and the nasal bone.
_Origin._—The lateral border of the nasal bone.
_Insertion._—The inner wall of the false nostril and the parietal lamina of the septal cartilage.
_Action._—To dilate the vestibule of the nasal cavity.
_Structure._—Fleshy.
_Relations._—Superficially, the skin, fascia, and levator labii superioris proprius; deeply, the parietal cartilage and false nostril.
_Blood-supply._—Facial artery.
_Nerve-supply._—Facial nerve.
5. =Dilatator naris inferior= (Pars ventralis m. lateralis nasi).—This is a similar but thicker muscle, which lies on the nasal process of the premaxilla.
_Origin._—The maxilla and the nasal process of the premaxilla.
_Insertion._—The cartilaginous prolongations of the turbinal bones and the inner wall of the false nostril.
_Action._—To rotate the turbinal outward and dilate the nostril and the vestibule of the nasal cavity.
_Structure._—Fleshy. A division into two or more parts may be recognized. A small part posteriorly is inserted into the cartilage of the dorsal turbinal bone, while the bulk of the muscle is inserted into the cartilage of the ventral turbinal bone. A few fibers also pass between the cornu of the alar cartilage and the outer wing of the nostril.
_Relations._—Superficially, the levator labii superioris proprius, the levator nasolabialis, and the lateral nasal artery; deeply, the maxilla, the premaxilla, and the anterior nasal branch of the infraorbital nerve.
_Blood-supply._—Facial artery.
_Nerve-supply._—Facial nerve.
MUSCLES OF THE EYELIDS
1. =Orbicularis oculi.=—This is a flat, elliptical, sphincter muscle, situated in and around the eyelids, the portion in the upper lid being much broader than that in the lower. The chief attachment is to the skin of the lids, but some bundles are attached to the palpebral ligament at the inner canthus and to the lacrimal bone. Its action is to close the lids.
2. =Corrugator supercilii.=—This is a very thin, small muscle, which arises over the root of the supraorbital process and spreads out in the upper eyelid, blending with the orbicularis. Its action is to assist in raising the upper lid or, especially in pathological conditions, to wrinkle the skin.
3. =Malaris.=—This is a very thin muscle, which varies much in different subjects. It extends from the fascia in front of the orbit to the lower lid. Its action is to depress the lower lid.
The foregoing muscles receive their blood-supply from the facial, transverse facial, supraorbital, and infraorbital arteries; the nerve-supply is derived from the facial nerve.
4. =Levator palpebræ superioris.=—This slender, flat muscle is almost entirely within the orbital cavity. It arises on the pterygoid crest, passes forward above the rectus oculi superior and below the lacrimal gland, and terminates in a thin tendon in the upper lid.
_Action._—To elevate the upper lid.
_Blood-supply._—Ophthalmic artery.
_Nerve-supply._—Oculomotor nerve.
MUSCLES OF MASTICATION
The muscles of this group are six in number in the horse. They arise chiefly from the upper jaw and the base of the cranium, and are all inserted into the mandible.
1. =Masseter.=—This muscle extends from the zygomatic arch and facial crest over the broad part of the mandibular ramus. It is semi-elliptical in outline.
_Origin._—By a strong tendon from the zygomatic arch and the facial crest.
_Insertion._—The outer surface of the broad part of the ramus of the mandible.
_Action._—Its action is to bring the jaws together. Acting singly, it also carries the lower jaw toward the side of the contracting muscle.
_Structure._—The superficial face of the muscle in its upper part is covered by a strong, glistening aponeurosis, and several tendinous intersections partially divide the muscle into layers. The fibers of the superficial layer take origin from the malar and maxilla only, and diverge somewhat to their insertion close to the thick ventral border of the lower jaw. The fibers of the deep layer arise from the entire area of origin, and pass straight to the border of the mandible; it will be noted that a small part, near the temporo-maxillary joint, is not covered by the superficial layer. The two layers are separable only above and behind; elsewhere they are fused.
_Relations._—Superficially, the skin and panniculus, the parotid gland, the transverse facial and masseteric vessels, and the facial nerve; deeply, the ramus of the mandible, the buccinator, depressor labii inferioris, and mylo-hyoideus muscles, the superior buccal glands, the buccinator nerve, and two large varicose veins which join the facial vein at the anterior edge of the muscle. The facial vessels and parotid duct run along the anterior edge of the muscle; the duct, however, bends forward about the middle of the border and leaves the muscle.
_Blood-supply._—Transverse facial and masseteric arteries.
_Nerve-supply._—Mandibular nerve.
2. =Temporalis.=—This muscle occupies the temporal fossa.
_Origin._—The rough part of the temporal fossa and the crests which surround it.
_Insertion._—The coronoid process of the mandible, which it envelops.
_Action._—Chiefly to raise the lower jaw, acting with the masseter and internal pterygoid muscles.
_Structure._—The surface of the muscle is covered with a glistening aponeurosis, and strong tendinous intersections are found in its substance. The inner edge of the muscle is quite thin, but as the fibers converge toward the much smaller area of insertion, the muscle becomes nearly an inch thick. It fuses somewhat with the masseter.
_Relations._—Superficially, the scutiform cartilage and anterior muscles of the external ear and the orbital fat; deeply, the temporal fossa and the deep temporal vessels and nerves.
_Blood-supply._—Superficial and deep temporal, and mastoid arteries.
_Nerve-supply._—Mandibular nerve.
3. =Pterygoideus internus= (s. medialis).—This muscle occupies a position on the inner surface of the ramus of the mandible similar to that of the masseter on the outside.
_Origin._—The crest formed by the pterygoid process of the sphenoid and the palate bone.