Part 25

_Insertion._—The concave inner surface of the broad portion of the ramus of the mandible, and the inner lip of the lower border.

_Action._—Acting together, to raise the lower jaw; acting singly, to produce also lateral movement of the jaw.

_Structure._—The muscle is capable of division into two parts. The principal part is superficial (internal), and its fibers are, for the most part, vertical in direction. It contains much tendinous tissue (septa). The smaller portion is external to the foregoing, and its fibers are directed downward and backward.

[Illustration:

FIG. 175.—SAGITTAL SECTION OF HEAD OF HORSE, SHOWING DEEP PTERYGO-MAXILLARY REGION, AND NASAL AND CRANIAL CAVITIES.

_1_, Cerebral compartment of cranial cavity; _2_, cerebellar compartment of same; _3_, tentorium osseum; _4_, tentorium cerebelli; _5_, sphenoidal sinus; _6_, hamulus of pterygoid bone-tendon of tensor palati cut off short at anterior border of hamulus; _7_, mylo-glossus. The olfactory mucous membrane is shaded. ]

_Relations._—Externally, the ramus of the mandible, the external pterygoid muscle, the inferior alveolar vessels and nerve, and the lingual and mylo-hyoid nerves; internally, the great cornu of the hyoid bone, the tensor palati, pterygo-pharyngeus, palato-pharyngeus, mylo-hyoideus, digastricus, and stylo-hyoideus muscles, the guttural pouch, the external maxillary vessels, the ninth and twelfth nerves, the submaxillary salivary gland, the submaxillary and parotid ducts, and the submaxillary and pharyngeal lymph glands.

_Blood-supply._—Internal maxillary, masseteric, and inferior alveolar arteries.

_Nerve-supply._—Mandibular nerve.

4. =Pterygoideus externus= (=s. lateralis=).—This muscle is considerably smaller than the preceding one, and is situated external to its upper part.

_Origin._—The external surface of the pterygoid process of the sphenoid bone.

_Insertion._—The inner surface of the neck and the inner part of the anterior border of the condyle of the mandible.

_Action._—Acting together, to draw the lower jaw forward; acting singly, to move the jaw laterally, _i. e._, toward the side opposite to the muscle acting. The latter action is due to the fact that the origin is nearer to the median plane than the insertion.

[Illustration:

FIG. 176.—SUBMAXILLARY AND LARYNGEAL REGIONS OF HORSE, AFTER REMOVAL OF SKIN AND PANNICULUS.

_c_, Mastoido-humeralis; _d_, sterno-cephalicus; _e_, omo-hyoideus and sterno-hyoideus; _h_, buccinator; _i_, depressor labii inferioris; _m_, masseter; _v_, stylo-maxillaris; _w_, mylo-hyoideus; _2_, posterior, _3_, anterior, border of external ear; _30′_, angle of jaw; _36_, submaxillary lymph glands; _37_, external maxillary vein; _39_, facial continuation of _37_; _40_, parotid duct; _44_, parotid gland; _45_, prominence of chin; _x_, wing of atlas. (After Ellenberger-Baum, Anat. für Künstler.) ]

_Structure._—The muscle is almost entirely fleshy, and the fibers are almost longitudinal in direction. Some of them are inserted into the edge of the interarticular meniscus.

_Relations._—Externally, the temporo-maxillary articulation and the temporalis muscle; internally, the internal pterygoid and tensor palati muscles. The internal maxillary artery crosses the lower face of the muscle and dips in between it and the tensor palati. The mandibular nerve lies on the ventral surface, and the buccinator nerve perforates the origin of the muscle.

_Blood-supply._—Internal maxillary and inferior alveolar arteries.

_Nerve-supply._—Mandibular nerve.

5. =Stylo-maxillaris= (s.-mandibularis) (M. jugulomandibularis).—This is a short muscle extending from the paramastoid or styloid process of the occipital bone to the posterior border of the lower jaw; it is covered by the parotid gland.

_Origin._—The paramastoid or styloid process of the occipital bone, in common with the posterior belly of the digastricus.

_Insertion._—The posterior border of the ramus of the mandible.

_Action._—To depress the lower jaw and open the mouth.

_Structure._—The muscle contains a good deal of tendinous tissue. It blends with the posterior belly of the digastricus.

_Relations._—Superficially, the parotid gland, the tendon of the sterno-cephalicus, and the fibrous expansion which connects it with the tendon of the mastoido-humeralis; deeply, the guttural pouch, the external carotid artery, the ninth and twelfth nerves, the pharynx, and the submaxillary gland.

_Blood-supply._—External carotid artery.

_Nerve-supply._—Facial nerve.

6. =Digastricus.=—This muscle is composed of two fusiform, flattened bellies, united by a round tendon.

_Origin._—The paramastoid or styloid process of the occipital bone, in common with the preceding muscle.

_Insertion._—The inner surface of the lower border of the mandible behind the symphysis.

_Action._—It assists in depressing the lower jaw and opening the mouth. If the mandible be fixed and both bellies contract, the hyoid bone and the base of the tongue are raised, as in the first phase of deglutition.

_Structure._—The posterior belly has the appearance of a branch detached from the inner surface of the stylo-maxillaris. It passes downward and forward, and is succeeded by a small rounded tendon. The latter perforates the tendon of insertion of the stylo-hyoideus, and is provided with a =synovial sheath=. The anterior belly is larger and terminates by thin tendinous bundles.

_Relations._—The posterior belly has practically the same relations as the stylo-maxillaris. The intermediate tendon is in contact externally with the internal pterygoid muscle, the submaxillary gland and duct, and the external maxillary artery. The anterior belly lies in the submaxillary space between the ramus of the jaw and the mylo-hyoideus muscle; the sublingual vessels run along its upper border.

_Blood-supply._—External carotid and sublingual arteries.

_Nerve-supply._—Facial and mandibular nerves.

THE HYOID MUSCLES

This group consists of eight muscles, one of which, the hyoideus transversus, is unpaired.

1. =Mylo-hyoideus.=—This muscle, together with its fellow, forms a sort of sling between the two rami of the lower jaw, in which the tongue is supported.

_Origin._—The inner surface of the alveolar border of the mandible.

_Insertion._—(1) A median fibrous raphé extending from the symphysis to the hyoid bone; (2) the lingual process and body of the hyoid bone.

_Action._—It raises the floor of the mouth, the tongue, and the hyoid bone.

_Structure._—Each muscle consists of a thin curved sheet, the fibers passing downward from their origin and then curving toward the median raphé. It is chiefly fleshy, and is thickest behind. The anterior superficial part of the muscle is termed the mylo-glossus.

_Relations._—On the superficial surface of the muscles are the ramus, the internal pterygoid and digastricus muscles, and the submaxillary lymph glands. The deep surface is in contact with the mucous membrane of the mouth, the stylo-glossus, hyo-glossus, and genio-hyoideus muscles, the sublingual gland and vessels, the submaxillary duct, and the lingual and hypoglossal nerves.

_Blood-supply._—Sublingual artery.

_Nerve-supply._—Mylo-hyoid branch of the mandibular nerve.

2. =Stylo-hyoideus.=—This is a slender, fusiform muscle, having a direction nearly parallel to that of the great cornu of the hyoid bone (Fig. 436).

_Origin._—The heel-like prominence on the proximal extremity of the great cornu of the hyoid bone.

_Insertion._—The anterior part of the thyroid cornu of the hyoid bone.

_Action._—It draws the base of the tongue and the larynx upward and backward.

_Structure._—It arises by a thin, short tendon, and has a fusiform belly. The tendon of insertion is perforated for the passage of the intermediate tendon of the digastricus, and at this point there is a small synovial sheath.

_Relations._—Superficially, the internal pterygoid muscle and the parotid gland; deeply, the guttural pouch, the pharynx, the external carotid and maxillary arteries, and the hypoglossal nerve.

_Blood-supply._—External carotid artery.

_Nerve-supply._—Facial nerve (stylo-hyoid branch).

3. =Occipito-hyoideus= (M. jugulo-hyoideus; occipito-styloideus).—This is a small triangular muscle, which lies in the space between the paramastoid process and the great cornu of the hyoid bone.

_Origin._—The paramastoid (styloid) process of the occipital bone.

_Insertion._—The proximal extremity and ventral edge of the great cornu of the hyoid bone.

_Action._—It carries the distal extremity of the great cornu backward and upward. Acting with the genio-hyoideus and digastricus, it raises the hyoid bone and the larynx.

_Structure._—The muscle is somewhat triangular, its fibers being longer as the ventral border is approached. It blends with the posterior belly of the digastricus.

_Relations._—Superficially, the parotid gland; deeply, the guttural pouch.

_Blood-supply._—Occipital artery.

_Nerve-supply._—Facial nerve.

4. =Genio-hyoideus.=—This is a long, spindle-shaped muscle, which lies under the tongue in contact with its fellow of the opposite side (Fig. 243).

_Origin._—The angle of union of the rami of the mandible.

_Insertion._—The extremity of the lingual process of the hyoid bone.

_Action._—It draws the hyoid bone and tongue forward.

_Structure._—The muscle arises by a short tendon, which is succeeded by the belly, composed of long bundles of parallel fibers.

_Relations._—Below, the mylo-hyoideus; above, the hyo-glossus, stylo-glossus, genio-glossus, the sublingual gland, submaxillary duct, and the lingual nerve.

_Blood-supply._—Sublingual artery.

_Nerve-supply._—Hypoglossal nerve.

5. =Kerato-hyoideus.=—This small triangular muscle lies in the space between the thyroid and small cornu, under cover of the hyo-glossus (Fig. 243).

_Origin._—The posterior edge of the small cornu and the adjacent part of the ventral border of the great cornu.

_Insertion._—The dorsal edge of the thyroid cornu.

_Action._—It raises the thyroid cornu and the larynx.

_Relations._—The muscle is crossed outwardly by the lingual artery.

_Blood-supply._—Lingual artery.

_Nerve-supply._—Glosso-pharyngeal nerve.

6. =Hyoideus Transversus.=—This is a thin, unpaired muscle, which extends transversely between the two small cornua of the hyoid bone.

_Attachments._—The small cornua close to the junction with the great cornua.

_Action._—When relaxed, its upper surface is concave; when it contracts, it elevates the root of the tongue.

_Structure._—Fleshy, composed of parallel transverse bundles.

_Blood-supply._—Lingual artery.

_Nerve-supply._—Glosso-pharyngeal nerve.

7. =Sterno-thyro-hyoideus=, and

8. =Omo-hyoideus.=—These are described with the muscles on the ventral surface of the neck.

THE FASCIÆ AND MUSCLES OF THE NECK

It is convenient to divide the muscles of the neck into ventral and lateral groups, the two lateral groups being separated from each other by the ligamentum nuchæ.

THE FASCIÆ OF THE NECK

The =superficial fascia= is in part two-layered, and contains the cervical portion of the panniculus. The fasciæ of the right and left sides are attached along the dorsal line of the neck to the ligamentum nuchæ, while along the ventral line they meet in a fibrous raphé. A deep layer is detached which passes underneath the panniculus, bridges over the jugular furrow, and crosses over the deep face of the mastoido-humeralis and omo-hyoideus to join the superficial layer. It again separates to pass under the cervical trapezius, and become attached to the ligamentum nuchæ. Along the ventral line a septum is detached which separates the sterno-cephalici. Two other layers in front of the shoulder inclose the prescapular lymph glands.

The =deep fascia= also forms two layers. The =superficial layer= is attached to the wing of the atlas and the lower edge of the trachelo-mastoideus and scalenus. Passing downward, it incloses the trachea, and, together with the deep layer, furnishes sheaths for the vagus and sympathetic nerves and the carotid artery. Passing upward it detaches septa between the extensor muscles of the spine. Anteriorly it covers the thyroid gland, the guttural pouch, the adjacent vessels and nerves and the larynx, and is attached to the mastoid process of the temporal bone and the thyroid cornu of the hyoid bone. Posteriorly, it is attached to the first rib and the cariniform cartilage of the sternum. The =deep layer= (prevertebral fascia) covers the ventral surface of the longus colli, and incloses the trachea and œsophagus. Anteriorly, it forms, with the corresponding layer of the opposite side, a septum between the guttural pouches; posteriorly, it becomes continuous with the endothoracic fascia. A =fascia propria= forms a tubular sheath around the trachea, inclosing also the recurrent nerves.

VENTRAL MUSCLES

This group consists of eleven pairs of muscles which lie almost entirely ventral to the vertebræ.

1. =Panniculus carnosus.=—The =cervical panniculus= (platysma myoides of man) has a fleshy origin on the cariniform cartilage (manubrium) of the sternum. it passes forward, outward, and upward, crossing over the sterno-cephalicus and jugular furrow obliquely. On reaching the surface of the mastoido-humeralis it adheres closely to this muscle, and soon thins out, to be continued over the splenius and trapezius by an aponeurosis which is difficult to remove from the latter muscle. Scattered bundles may be traced on the ventral surface of the neck to the facial portion. The right and left muscles meet at a ventral median raphé in pennate fashion.

[Illustration:

FIG. 177.—ANTERO-LATERAL VIEW OF MUSCLES AND SKELETON OF HORSE.

_a_, Trapezius; _c_, mastoido-humeralis; _d_, sterno-cephalicus; _f_, long head of triceps; _f′_, external head of triceps; _g_, anterior superficial pectoral muscle; _g′_, posterior superficial pectoral; _h′_, anterior deep pectoral; _v_, cervical panniculus; _z_, supraspinatus; _29_, omo-hyoideus; _30_, sterno-thyro-hyoideus; _31_, jugular vein; _32_, cephalic vein; _1_, scapula; _1′_, cartilage of scapula; _2_, spine of scapula; _4_, shaft of humerus; _4′_, external epicondyle; _5_, external tuberosity of humerus; _6_, deltoid tuberosity; _14_, ventral border (“keel”) of sternum; _14′_, cariniform cartilage; l.R., first rib. (After Ellenberger-Baum, Anat. für Künstler.) ]

2. =Mastoido-humeralis.=—This is described on p. 252.

3. =Sterno-cephalicus= (Sterno-maxillaris s.-mandibularis).[42]—This is a long, narrow muscle, extending along the ventral and lateral aspects of the trachea from the sternum to the angle of the jaw. It forms the ventral boundary of the jugular furrow.

_Origin._—The cariniform cartilage of the sternum.

_Insertion._—The posterior border of the lower jaw.

_Action._—Acting together, to flex the head and neck; acting singly, to incline the head and neck to the side of the muscle contracting.

_Structure._—The two muscles are fused at their origin, which is fleshy. Near the middle of the neck they separate, and, becoming thinner, each muscle passes under the parotid gland and terminates in a flat tendon.

_Relations._—Superficially, the cervical panniculus; deeply, the sterno-thyro-hyoideus and omo-hyoideus muscles. The upper edge of the muscle is related to the jugular vein, which lies in the jugular furrow. The carotid artery, the vagus, sympathetic, and recurrent nerves also lie along the upper edge at the root of the neck. The tendon passes under the submaxillary vein and the parotid gland, having the submaxillary gland and stylo-maxillaris muscle on its inner side.

_Blood-supply._—Carotid artery.

_Nerve-supply._—Ventral branches of the spinal accessory and first cervical nerves.

4. =Sterno-thyro-hyoideus= (Sterno-thyroideus et sterno-hyoideus).—This is a long, slender, digastric muscle, applied to the ventral surface of the trachea and its fellow of the opposite side.

_Origin._—The cariniform cartilage of the sternum.

_Insertion._—(1) The external surface of the thyroid cartilage of the larynx; (2) the body of the hyoid bone.

_Action._—To depress and retract the hyoid bone, the base of the tongue, and the larynx, as in deglutition. It may also fix the hyoid bone when the depressors of the tongue are acting, as in suction.

_Structure._—The origin of the muscle is fleshy, and as far as the middle of the neck it blends with its fellow. The common belly is then interrupted by a tendon, or sometimes two tendons, from which arise three or four fleshy bands. The lateral bands diverge to reach their insertion into the thyroid cartilage; while the inner bands, closely applied to each other and blending with the omo-hyoideus, pass straight forward to reach the ventral surface of the hyoid bone.

_Relations._—At the root of the neck the common belly has the sterno-cephalicus below, and the carotid arteries and recurrent nerves above. Further forward the trachea becomes the upper relation, and near the head the skin and fascia, the lower one.

_Blood-supply._—Carotid artery.

_Nerve-supply._—Ventral branch of the first cervical nerve.

5. =Omo-hyoideus= (Subscapulo-hyoideus).—This is a thin, ribbon-like muscle, almost entirely fleshy, which crosses the trachea very obliquely under cover of the mastoido-humeralis.

_Origin._—The subscapular fascia close to the shoulder joint.

_Insertion._—The body of the hyoid bone, in common with the hyoid branch of the preceding muscle.

_Action._—To depress the hyoid bone.

_Structure._—The muscle is composed of parallel fleshy fibers, except at its origin, where it has a thin tendon.

_Relations._—In the first part of its course the muscle passes forward between the supraspinatus, anterior deep pectoral, mastoido-humeralis (outwardly), and the scalenus (inwardly). It is intimately adherent to the mastoido-humeralis. In the middle of the neck it is related superficially to the mastoido-humeralis, sterno-cephalicus, and the jugular vein; deeply, to the rectus capitis anterior major, the carotid artery, the vagus, sympathetic, and recurrent nerves, the trachea, and, on the left side, the œsophagus. In its anterior part the muscle blends with the hyoid part of the sterno-thyro-hyoideus, the two covering the thyroid portion of the latter muscle, the thyroid gland, and the ventral face of the larynx.

_Blood-supply._—Carotid and inferior cervical arteries.

_Nerve-supply._—Ventral branches of the cervical nerves.

6. =Scalenus= (M. scalenus primæ costæ).—This muscle is deeply situated on the side of the posterior half of the neck. It is composed of two parts, between which the roots of the brachial plexus of nerves emerge.

_Origin._—The anterior border and outer surface of the first rib.

_Insertion._—(1) The dorsal (smaller) portion is attached to the transverse process of the seventh cervical vertebra; (2) the ventral portion is attached to the transverse processes of the sixth, fifth, and fourth cervical vertebræ.

_Action._—The neck is flexed or inclined laterally, according as the muscles act together or singly. If the neck be the fixed point, the muscle may have a respiratory action by pulling forward or fixing the first rib.

_Structure._—The dorsal portion is composed of three or four fleshy bundles.[43] The ventral portion, which is much larger, is almost entirely fleshy, and not so divided.

_Relations._—Superficially, the anterior deep pectoral, mastoido-humeralis, and omo-hyoideus muscles, the phrenic nerve, and the other branches of the brachial plexus; deeply, the vertebræ, the longus colli and intertransversales muscles, the œsophagus (on the left side), the trachea (on the right side), the vertebral vessels, the vagus, sympathetic, and recurrent nerves. The roots of the brachial plexus form a flat anastomosis, which lies between the two portions of the muscle. The brachial vessels cross the ventral edge close to the first rib.

_Blood-supply._—Carotid, vertebral, and inferior cervical arteries.

_Nerve-supply._—Ventral branches of the cervical nerves.

7. =Rectus capitis anterior major= (M. longus capitis).—This is the largest of the three special flexors of the head, and lies along the ventro-lateral surface of the anterior cervical vertebræ and the base of the cranium.

_Origin._—The transverse processes of the fifth, fourth, and third cervical vertebræ.

_Insertion._—The tubercles at the junction of the basilar process of the occipital bone with the body of the sphenoid.

_Action._—Acting together, to flex the head; acting singly, to incline it to the same side also.

_Structure._—The origin of the muscle is by fleshy digitations. The belly increases in size by the union of these digitations, reaching its maximum at the axis. It then diminishes, passes toward the median plane, and terminates on a rounded tendon.

_Relations._—Superficially, the mastoido-humeralis, omo-hyoideus, and rectus capitis anticus minor, the submaxillary gland, the carotid artery (which lies along the lower border), the occipital and internal carotid arteries, and the tenth, eleventh, and sympathetic nerves; deeply, the vertebræ, the longus colli, intertransversales, and the small straight muscle. The terminal part of the muscle lies in contact with its fellow above the pharynx and between the guttural pouches.

_Blood-supply._—Carotid, vertebral, and occipital arteries.

_Nerve-supply._—Ventral branches of the cervical nerves.

8. =Rectus capitis anterior minor= (M. rectus capitis ventralis).—This is a small muscle which lies under cover of the preceding one.

_Origin._—The ventral arch of the atlas.

_Insertion._—The basilar process, close to the preceding muscle.

_Action._—To flex the occipito-atlantal articulation.

_Structure._—Fleshy.

_Relations._—Below, to the preceding muscle; above, to the atlas, occipito-atlantal articulation, and the basilar process; externally, to the rectus capitis lateralis and the guttural pouch.

_Blood-supply._—Occipital artery.

_Nerve-supply._—Ventral branch of the first cervical nerve.

9. =Rectus capitis lateralis.=—This is a still smaller, entirely fleshy muscle, which lies for the most part under the small oblique muscle.

_Origin._—The atlas, external to the preceding muscle.

_Insertion._—The paramastoid or styloid process of the occipital bone.

_Action._—The same as the preceding muscle.

_Structure_.—Fleshy.

_Relations._—Superficially, the small oblique muscle, the occipital vessels, and the ventral branch of the first cervical nerve.

_Blood-supply._—Occipital artery.

_Nerve-supply._—Ventral branch of the first cervical nerve.

10. =Longus colli.=—This muscle covers the ventral surface of the vertebræ, from the sixth thoracic to the atlas, and is united with its fellow. It consists of two portions, thoracic and cervical.

_Origin._—(1) Thoracic portion, the bodies of the first six thoracic vertebræ; (2) cervical portion, the transverse processes of the cervical vertebræ.

_Insertion._—(1) Thoracic portion, the bodies and transverse processes of the last two cervical vertebræ; (2) cervical portion, the bodies of the cervical vertebræ and the tubercle on the ventral surface of the atlas.

_Action._—To flex the neck.

_Structure._—The muscle is composed of a succession of bundles. The largest of these constitute the thoracic part of the muscle, which has a strong tendon inserted into the last two cervical vertebræ. A bursa is interposed between the tendon and the spine at the first costo-vertebral articulation. The cervical portion consists of a number of smaller bundles, each of which passes from its origin on the transverse process of one vertebra forward and inward to its insertion into a vertebra further forward. The most anterior bundle is inserted by a strong tendon into the ventral tubercle of the atlas.

_Relations._—The principal relations of the two muscles in the thorax are: ventrally, the pleura, and, further forward, the trachea and œsophagus; dorsally, the vertebræ and the costo-vertebral joints; laterally, the dorsal, superior cervical, and vertebral vessels, the sympathetic nerve, and the thoracic roots of the brachial plexus. In the neck, important relations are: ventrally, the trachea and œsophagus, the carotid artery, the vagus, sympathetic and recurrent nerves; dorsally, the vertebræ and, in the middle third of the neck, the intertransversales muscles; laterally, the scalenus, the rectus capitis anticus major, and the intertransversales (in the anterior third). The terminal part of the muscle is separated from the trachea by the œsophagus, which is here median in position.

_Blood-supply._—Subcostal and vertebral arteries.

_Nerve-supply._—Ventral branches of the spinal nerves.

11. =Intertransversales colli= (Mm. intertransversarii cervicis).—These are six fasciculi which occupy the spaces between the lateral aspects of the vertebræ and the transverse and articular processes. There is thus a bundle for each intervertebral articulation except the first. Each bundle consists of a dorsal and ventral portion.

_Attachments._—The dorsal bundles pass from transverse process to articular process; the ventral bundles extend between adjacent transverse processes.

_Action._—To bend the neck laterally.

_Structure._—They contain strong tendinous intersections.

_Relations._—Superficially, the mastoido-humeralis, rectus capitis anterior major, complexus, trachelo-mastoideus, splenius, scalenus, and longissimus muscles; deeply, the vertebræ, the longus colli muscle, and the vertebral vessels. The muscles are perforated by branches of these vessels and by the primary branches of the cervical nerves.

_Blood-supply._—Vertebral artery.

_Nerve-supply._—The cervical nerves.

LATERAL MUSCLES

This group consists of twelve pairs of muscles arranged in layers.

FIRST LAYER

1. =Trapezius cervicalis.=—Described on p. 250.

SECOND LAYER

2. =Rhomboideus cervicalis.=—Described on p. 251.

3. =Serratus magnus= (M. serratus cervicis).—Described with the thoracic part on p. 254.

THIRD LAYER

4. =Splenius.=—This is an extensive, flat, triangular muscle, partly covered by the preceding three muscles.

_Origin._—The second, third, and fourth thoracic spines by means of the dorso-scapular ligament, and the funicular portion of the ligamentum nuchæ.

_Insertion._—The occipital crest, the mastoid process, the wing of the atlas, and the transverse processes of the third, fourth, and fifth cervical vertebræ.

_Action._—Acting together, to elevate the head and neck; acting singly, to incline the head and neck to the side of the muscle acting.

_Structure._—The muscle arises in the withers from the anterior part of the dorso-scapular ligament, which also affords attachment to the rhomboideus, serratus anticus, and complexus muscles. The fibers pass upward and forward toward the head and the first cervical vertebra. The insertion on the occipital bone and the mastoid process is by means of a thin aponeurosis common to the splenius, trachelo-mastoideus, and mastoido-humeralis. The atlantal insertion is by a strong, flat tendon, also in common with these muscles. The remaining insertions are fleshy digitations.

_Relations._—Superficially, the skin and fascia, the trapezius rhomboideus, cervicalis, serratus magnus, and posterior auricular muscles; deeply, the complexus, trachelo-mastoideus, and longissimus muscles.

_Blood-supply._—Deep cervical and dorsal arteries.

_Nerve-supply._—Dorsal branches of the last six cervical nerves.

FOURTH LAYER

5. =Trachelo-mastoideus= (M. longissimus capitis et atlantis).—This muscle consists of two parallel, fusiform portions. It lies between the deep face of the splenius and the ventral part of the complexus.

_Origin._—(1) The transverse processes of the first two thoracic vertebræ; (2) the articular processes of the cervical vertebræ.

_Insertion._—(1) The mastoid process; (2) the wing of the atlas.

[Illustration:

FIG. 178.—SUPERFICIAL MUSCLES OF HORSE.