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.
Fig. 178.—Superficial Muscles of Horse.
The panniculus, except the cervical part, has been removed. a, Trapezius cervicalis; a′, trapezius thoracalis; c, c′, mastoido-humeralis; d, sterno-cephalicus; e, deltoid; f, long head of triceps; f′, external head of triceps; g, anterior superficial pectoral; h, posterior deep pectoral; h′, anterior deep pectoral; i, serratus thoracis; i′, serratus cervicis; k, latissimus dorsi; l, obliquus abdominis externus; l′, aponeurosis of l; m, serratus posticus; m′, lumbo-dorsal fascia; o, tensor fasciæ latæ; o′, fascia lata; o″, gluteus superficialis; q, q′, q″, biceps femoris; r, semitendinosus; s, sacro-coccygeus superior; t, sacro-coccygeus lateralis; u, coccygeus; v, cervical panniculus; w, splenius; x, rhomboideus; y, tendon of trachelo-mastoideus and mastoido-humeralis; z, suprasinatus; X, wing of atlas; 2, spine of scapula; 4′, external epicondyle of humerus; 6, deltoid tuberosity; 8, olecranon; 16, external angle of ilium; 20, patella; 21′, external condyle of tibia. (After Ellenberger-Baum, Anat. für Künstler.)
Action.—Acting together, to extend the head and neck; acting singly, to flex the head and neck laterally or to rotate the atlas.
Structure.—The origin from the thoracic vertebræ is by aponeurotic slips which blend with the complexus. The succeeding fleshy portion, in passing along the neck, receives fasciculi from each of the cervical vertebræ except the first two. The dorsal division of the muscle is inserted into the mastoid process by a flat tendon which fuses with that of the splenius; the ventral division is inserted into the wing of the atlas by a ribbon-like tendon in common with the splenius and mastoido-humeralis.
Relations.—Superficially, the splenius muscle and dorsal branches of the cervical nerves; deeply, the complexus, the spinalis colli, and the oblique muscles of the head. The superior cervical vessels cross the deep face of the muscle obliquely at the level of the sixth and seventh cervical vertebræ.
Blood-supply.—Vertebral and superior cervical arteries.
Nerve-supply.—Dorsal branches of the last six cervical nerves.
6. Complexus (M. semispinalis capitis).—This is a large triangular muscle which lies chiefly on the ligamentum nuchæ, under cover of the splenius and trachelo-mastoideus muscles.
Origin.—(1) The second, third, and fourth thoracic spines, in common with the splenius and serratus anticus; (2) the transverse processes of the first six or seven thoracic vertebræ; (3) the articular processes of the cervical vertebræ.
Insertion.—The posterior surface of the occipital bone, external to the funicular portion of the ligamentum nuchæ.
Action.—It is the chief extensor of the head and neck. Acting singly, the muscle inclines the head to the same side.
Structure.—The origin of the muscle at the withers is aponeurotic. In the neck the bundles arising from the articular processes run obliquely upward and forward, giving the lower part of the muscle a distinct pennate character. The upper part of the belly is crossed obliquely by four or five tendinous intersections. The insertion is by a strong tendon.
Relations.—Superficially, the rhomboideus, serratus magnus, splenius, and trachelo-mastoideus muscles; deeply, the ligamentum nuchæ, the multifidus cervicis, longissimus, and the oblique and posterior straight muscles of the head, the deep or superior cervical vessels, and the dorsal cutaneous branches of the cervical nerves.
Blood-supply.—Deep cervical, vertebral, and occipital arteries.
Nerve-supply.—Dorsal branches of the last six cervical nerves.
7. Multifidus cervicis (M. semispinalis colli; transverse spinous muscle of neck).—This muscle lies on the arches of the last five cervical vertebræ. It consists of five or six segments.
Origin.—The articular processes of the last five (or four) cervical and the first thoracic vertebræ.
Insertion.—The spinous processes of the cervical vertebræ.
Fig. 179.—Deeper Muscles of Horse.
f, Long head of triceps; f′, external head of triceps; g, anterior superficial pectoral; h, posterior deep pectoral; h′, anterior deep pectoral; i, serratus thoracis; i′, serratus cervicis; l, obliquus abdominis externus, and l′, its aponeurosis, the posterior part of which has been removed; m, serratus posticus; p, gluteus medius; r, semitendinosus; s, sacro-coccygeus superior; t, sacro-coccygeus lateralis; u, coccygeus; v′, biceps brachii; x, rhomboideus; y, y′, trachelo-mastoideus; z, supraspinatus; z′, infraspinatus; 1, cartilage of scapula; 2, spine of scapula; 5, external tuberosity of humerus; 6, deltoid tuberosity; 8, olecranon; 16, external angle of ilium; 19, trochanter major; 20, patella; 21′, external condyle of tibia; 26, articular processes of cervical vertebræ; 27, parotido-auricularis; 28, vastus externus; 28′, rectus femoris; 28″, trochanter tertius; 29, semimembranosus; 30, gastrocnemius; 31, sacro-sciatic ligament; 32, omo-hyoideus; 33, complexus; 34, rectus capitis anterior major; 35, spinalis dorsi; 36, longissimus dorsi; 37, transversalis costarum; 38, teres minor; 39, brachialis; 40, external intercostal; 41, obliquus abdominis internus; 42, iliacus; 43, transversus abdominis. (After Ellenberger-Baum, Anat. für Künstler.)
Action.—Acting together, to extend the neck; acting singly, to flex and rotate the neck to the opposite side.
Structure.—The muscle is composed of two sets of bundles. The superficial bundles are directed obliquely forward and inward, each passing from an articular process to the spine of the preceding vertebra. The deep bundles are shorter and run straight from an articular process to that of the preceding vertebra.
Relations.—Superficially, the complexus, longissimus, trachelo-mastoideus, and great oblique muscles; deeply, the spinalis muscle, the ligamentum nuchæ, and the arches of the vertebræ.
Blood-supply.—Deep cervical and vertebral arteries.
Nerve-supply.—Dorsal branches of the last six cervical nerves.
8. Spinalis.—Described with the longissimus on p. 237.
9. Obliquus capitis posterior (s. caudalis) (great oblique muscle of the head).—This is a strong, quadrilateral muscle, which covers the dorso-lateral aspect of the atlas and axis.
Origin.—The side of the spine and the posterior articular process of the axis.
Insertion.—The dorsal surface of the wing of the atlas.
Action.—Chiefly to rotate the atlas, and with it the head, to the same side; also to assist in extending and fixing the atlanto-axial joint.
Structure.—The muscle is composed almost entirely of parallel fleshy fibers directed obliquely forward and outward.
Relations.—Superficially, the skin, the splenius, complexus, trachelo-mastoideus, and mastoido-humeralis muscles; deeply, the arch and spine of the axis, the wing of the atlas, the atlanto-axial joint, the multifidus cervicis, the posterior straight muscles, the occipital and vertebral vessels, and the first and second cervical nerves. The terminal part of the vertebral artery joins the posterior branch of the occipital artery under cover of the muscle.
Blood-supply.—Occipital and vertebral arteries.
Nerve-supply.—Dorsal branch of the second cervical nerve.
10. Obliquus capitis anterior (s. cranialis) (small oblique muscle of the head).—A short, thick, quadrilateral muscle which lies on the side of the occipito-atlantal articulation.
Origin.—The anterior edge of the wing of the atlas and the fossa atlantis.
Insertion.—The styloid process and crest of the occipital bone, and the mastoid process.
Action.—Acting together, to extend the head on the atlas; acting singly, to flex the head laterally.
Structure.—The muscle contains a good deal of tendinous tissue. The direction of its fibers is forward, upward, and inward.
Relations.—Superficially, the posterior auricular muscles, artery, and nerve, the aponeurosis of the splenius, trachelo-mastoideus and mastoido-humeralis, and the parotid gland; deeply, the straight muscles, the complexus, the occipito-hyoideus, the occipito-atlantal articulation, branches of the occipital artery, and a branch of the occipital nerve.
Blood-supply.—Occipital artery.
Nerve-supply.—Dorsal branch of the first cervical nerve.
11. Rectus capitis posterior major (s. dorsalis major) (great posterior straight muscle of the head).—This muscle extends from the axis to the occipital bone, in contact with the ligamentum nuchæ.
Origin.—The edge of the spinous process of the axis.
Insertion.—The occipital bone, below and external to the complexus and ligamentum nuchæ.
Action.—To extend the head.
Fig. 180.—Deepest Layer of Muscles of Neck of Horse.
a, Obliquus capitis anterior; b, obliquus capitis posterior; c, rectus capitis lateralis; d, rectus capitis anterior minor; e, rectus capitis anterior major; f, longus colli; g, g′, scalenus; h, transversalis costarum; i, longissimus dorsi; k, spinalis et semispinalis; l, multifidus dorsi; m, multifidus cervicis; n, intertransversales; o, o′, rectus capitis posterior major; p, rectus capitis posterior minor; q, tendon of insertion of complexus; 1, lamellar part, 1′, funicular part of ligamentum nuchæ; 2, occipital crest; 3, paramastoid process; 4, edge of wing of atlas; 5, transverse, and 6, articular, processes of cervical vertebræ; 7, nerves of brachial plexus (cut); 8, first rib. (Ellenberger-Baum, Anat. d. Haustiere.)
Structure.—The muscle is fleshy and may be divided into two parallel portions, superficial and deep. The former blends somewhat with the terminal part of the complexus. The deep portion may be termed the rectus capitis posterior medius.
Relations.—Superficially, the anterior oblique, splenius, and complexus; internally, the ligamentum nuchæ; deeply, the atlas, the occipito-atlantal articulation, and the rectus capitis posterior minor. The dorsal branch of the first cervical nerve appears between this muscle and the small oblique.
Blood-supply.—Occipital artery.
Nerve-supply.—Dorsal branch of the first cervical nerve.
12. Rectus capitis posterior minor (s. dorsalis minor) (small posterior straight muscle of the head).—This small muscle lies under cover of the preceding.
Origin.—The dorsal surface of the atlas.
Insertion.—The occipital bone beneath the preceding muscle.
Action.—To assist the preceding muscle.
Structure.—It is fleshy and varies a good deal in volume, being sometimes small and difficult to recognize.[44] On the other hand, it is sometimes double.
Relations.—Superficially, the preceding muscle and the small oblique; deeply, the atlas and the occipito-atlantal articulation.
Blood-supply.—Occipital artery.
Nerve-supply.—Dorsal branch of the first cervical nerve.
The Fasciæ and Muscles of the Back and Loins
The superficial fascia presents no special features. The lumbo-dorsal fascia (Fascia lumbo-dorsalis) closely invests the muscles, but is easily stripped off the longissimus. It is attached medially to the supraspinous ligament and the spinous processes of the vertebræ; laterally, it divides into two layers. The superficial layer is practically the aponeurosis of the latissimus dorsi. The deep layer gives origin to the serratus anticus and posticus, the lumbar part of the obliquus externus abdominis, the transversus abdominis, and the retractor costæ. Its lateral edge curves under the longissimus and is attached to the ribs and lumbar transverse processes. Posteriorly, it is continuous with the gluteal fascia. At the withers it forms an important structure, the dorso-scapular ligament. This is a strong tendinous sheet, attached to the third, fourth, and fifth thoracic spines. Its upper part is very thick and gives origin by its superficial surface to the rhomboideus dorsi, by its anterior part to the splenius. The lower part is thin and elastic, and furnishes numerous lamellæ which intersect the scapular part of the serratus and are attached with it to the scapula. Three lamellæ are detached from the deep face of the ligament. The deepest of these passes between the longissimus and spinalis and is attached to the transverse processes of the first seven thoracic vertebræ; it gives attachment to the complexus. The middle one dips in between the longissimus and transversalis costarum. The superficial one gives origin to the serratus anticus. A strong fascial layer, the ilio-lumbar ligament, extends from the last rib to the external angle of the ilium.
There are nine pairs of muscles in this region, arranged in four layers.
First Layer
1. Trapezius thoracalis.
2. Latissimus dorsi.
Second Layer
3. Rhomboideus thoracalis.
The foregoing are described with the other muscles which attach the thoracic limb to the trunk (p. 251).
4. Serratus anticus (M. serratus dorsalis inspiratorius).—This is a thin quadrilateral muscle, named from its toothed ventral border. It lies beneath the rhomboideus, serratus magnus, and latissimus dorsi.
Origin.—The lumbo-dorsal fascia and dorso-scapular ligament.
Insertion.—The external surfaces of the fifth to the eleventh or twelfth ribs inclusive.
Action.—To draw the ribs on which it is inserted forward and outward, thus assisting in inspiration.
Structure.—The muscle arises by means of a thin aponeurosis which blends with the lumbo-dorsal fascia and the aponeurosis of the latissimus dorsi. The muscle-fibers pass downward and backward to be attached to the ribs by seven or eight digitations below the outer edge of the transversalis costarum.
Relations.—Superficially, the rhomboideus, serratus magnus, latissimus dorsi, and serratus posticus; deeply, the longissimus dorsi, transversalis costarum, external intercostal muscles, and the ribs.
Blood-supply.—Intercostal arteries.
Nerve-supply.—Thoracic nerves.
5. Serratus posticus (M. serratus dorsalis exspiratorius).—This muscle resembles the preceding one, which it partly covers.
Origin.—The lumbo-dorsal fascia.
Insertion.—The outer surfaces of the last seven or eight ribs.
Action.—To draw the ribs backward, thus assisting in expiration.
Structure.—Similar to the preceding muscle. The fibers are directed downward and forward and terminate in seven or eight digitations, one or two of which cover the posterior teeth of the anticus. The aponeurosis blends with that of the latissimus dorsi.
Relations.—Superficially, the latissimus dorsi and external oblique; deeply, the longissimus dorsi, transversalis costarum, external intercostals, serratus anticus, and the ribs.
Blood-supply.—Intercostal and lumbar arteries.
Nerve-supply.—Thoracic nerves.
Third Layer
6. Transversalis costarum (M. ilio-costalis).—This long, segmental muscle extends, as its name indicates, across the series of ribs, in contact with the outer edge of the longissimus dorsi.
Origin.—(1) The transverse processes of the second and third lumbar vertebræ and the deep layer of the lumbo-dorsal fascia. (2) The anterior borders and external surface of the last fifteen ribs.[45]
Insertion.—The posterior borders of the ribs and the transverse processes of the last two or three cervical vertebræ.
Action.—Chiefly to depress and retract the ribs and so help in expiration. Acting together, they may assist in extending this spine, acting singly in inclining it laterally.
Structure.—This muscle presents a distinct segmental arrangement. It is composed of a series of bundles, the fibers of which are directed forward and a little downward and outward. From these are detached two sets of tendons. The superficial tendons spring from the outer edge of the muscle. They are flat and are about half an inch in width. Each crosses two or three intercostal spaces, to be inserted on the posterior border of a rib. The deep tendons are detached from the dorsal part of the deep face of the muscle. Each passes backward across one or two intercostal spaces to its origin on the anterior border or outer surface of a rib. Small bursæ may be found between the ribs and tendons.
Relations.—Superficially, the dorsal serrati and the complexus; deeply, the external intercostals and the ribs. The lumbar origin is covered by the longissimus. The superior (deep) cervical and dorsal vessels cross the surface of the muscle at the first and second intercostal spaces respectively, and branches of the intercostal vessels and nerves emerge between the transversalis and longissimus; here a fascial layer dips in between the two.
Blood-supply.—Intercostal arteries.
Nerve-supply.—Dorsal branches of the thoracic nerves.
7. Longissimus.[46]—This is the largest and longest muscle in the body. It extends from the sacrum and ilium to the neck, filling up the space between the spinous processes medially and the lumbar transverse processes and the upper ends of the ribs below; consequently it has the form of a three-sided prism.
Origin.—(1) The internal angle, crest, and adjacent part of the ventral surface of the ilium; (2) the first three sacral spines; (3) the lumbar and thoracic spines and the supraspinous ligament.
Insertion.—(1) The lumbar transverse and articular processes; (2) the thoracic transverse processes; (3) the spinous and transverse processes of the last four cervical vertebræ; (4) the outer surfaces of the ribs, except the first.
Action.—Acting with its fellow, it is the most powerful extensor of the back and loins; by its cervical attachment it assists in extending the neck. By its costal attachment it may also assist in expiration. Acting singly, it flexes the spine laterally.
Fig. 181.—Right Portion of Cross-section of Back of Horse. Section is Cut Through Seventh Thoracic Vertebra.
a, Ligamentum nuchæ; b, trapezius muscle; c, cartilage of scapula; d, latissimus dorsi; e, panniculus carnosus; f, rhomboideus dorsi; g, serratus magnus; h, serratus anticus; h′, lumbo-dorsal fascia, which divides below into three layers; i, transversalis costarum; k, levator costæ; k′, internal intercostal muscle; l, longissimus dorsi; m, m′, seventh thoracic vertebra; n, head of eighth thoracic vertebra; o, head of eighth rib; p, seventh rib; r, intercostal artery and nerve; s, skin. The fasciæ are indicated by dotted lines. (After Ellenberger, in Leisering’s Atlas.)
Structure.—This is quite complex. The posterior part of the muscle is greatly developed and constitutes the common mass of the loins. This is covered by a strong aponeurosis which blends with the supraspinous and sacro-iliac ligaments, and is attached to the crest and inner (sacral) angle of the ilium and the first and second sacral spines; it furnishes origin to the lumbar portion of the middle gluteus. In its course further forward the muscle receives fasciculi from the lumbar and thoracic spines, but diminishes somewhat in volume. At the withers it divides into two parts. The dorsal division (spinalis et semispinalis), reinforced by bundles from the first four thoracic spines, passes forward under the complexus to be inserted into the spines of the last four cervical vertebræ. The ventral division passes forward and downward underneath the serratus magnus to be inserted into the ribs and the transverse processes of the last four cervical vertebræ. Three sets of fasciculi may be distinguished, viz.: (1) spinal, which are superficial and internal; (2) transverse, attached to the transverse and articular processes, which are internal and deep; (3) costal, which are external.
Relations.—Superficially, the middle gluteus, the lumbo-dorsal fascia, the latissimus dorsi, dorsal serrati, serratus magnus, and complexus; deeply, the multifidus, intertransversales, external intercostals, levatores costarum, the ligamentum nuchæ, and its fellow of the opposite side (in the neck).
Blood-supply.—Dorsal, superior cervical, intercostal, and lumbar arteries.
Nerve-supply.—Dorsal branches of the thoracic and lumbar nerves.
8. Multifidus dorsi (semispinalis of the back and loins).—This is a long segmental muscle which covers the sides of the spinous processes of the vertebræ from the sacrum to the neck.
Origin.—(1) The lateral portion of the sacrum; (2) the articular processes of the lumbar vertebræ; (3) the transverse processes of the thoracic vertebræ.
Insertion.—The spinous processes of the first two sacral, the lumbar, thoracic, and last cervical vertebræ.
Action.—Acting with its fellow, it is an extensor of the spine; acting singly, it flexes it laterally.
Structure.—It is composed of a series of bundles which are directed obliquely forward and upward. Each fasciculus passes over several vertebræ to its insertion. In the posterior part of the series the bundles cross two or three vertebræ and are inserted into the summits of the spines. Anteriorly, the bundles have a more horizontal direction, cross three to five vertebræ, and are inserted into the sides of the spines considerably below their summits. A further complication consists in the fusion of several bundles into a common insertion.
Relations.—Superficially, the longissimus; deeply, the vertebral spines.
Blood-supply.—Intercostal and lumbar arteries.
Nerve-supply.—Dorsal branches of the thoracic and lumbar nerves.
9. Intertransversales lumborum.—These are very thin muscular and tendinous strata, which occupy the spaces between the transverse processes of the lumbar vertebræ except the fifth and sixth.
Action.—To assist in flexing the loins laterally.
Relations.—Superficially, the longissimus; deeply, the quadratus lumborum.
Blood-supply.—Lumbar arteries.
Nerve-supply.—Lumbar nerves.
The Fascia and Muscles of the Tail
The muscles of the tail are inclosed in the strong coccygeal fascia, which is continuous in front with the gluteal fascia and blends with the lateral sacro-iliac ligament. At the root of the tail it is loosely attached to the subjacent muscles, but further back it is intimately adherent to them. From its deep face are detached septa which pass between the muscles to become attached to the vertebræ.
1. Coccygeus (Compressor coccygis; ischio-coccygeus).—This is a flat, triangular muscle which lies chiefly between the sacro-sciatic ligament and the rectum.
Origin.—The pelvic surface of the sacro-sciatic ligament near the ischiatic spine.
Insertion.—The first four coccygeal vertebræ and the coccygeal fascia.
Action.—Acting together, to depress (flex) the tail, compressing it over the perineum; acting singly, to depress and incline it to the same side.
Structure.—The origin of the muscle is aponeurotic. Becoming fleshy, its fibers pass upward and backward and divide into two layers. The outer layer is attached to the vertebræ, the inner to the fascia; included between the two lie the intertransversales. When the tail is raised, the ventral edges of the muscles produce a distinct ridge at either side of the anus.
Relations.—Outwardly, the sacro-sciatic ligament and the semimembranosus; inwardly, the rectum and the sacro-coccygeus ventralis muscle. The internal pudic artery crosses the outer face of the origin of the muscle.
2. Sacro-coccygeus superior (M. sacro-coccygeus dorsalis medialis; erector s. extensor coccygis).—This muscle lies along the dorso-median aspect of the tail, in contact with its fellow.
Origin.—The last three sacral spines and some of the coccygeal spines.
Insertion.—The dorsal surface of the coccygeal vertebræ.
Action.—Acting together, to elevate (extend) the tail; acting singly, to elevate and incline it laterally.
Structure.—The muscle has a strong rounded belly. It is inserted by means of short tendons which fuse with those of the next muscle.
Relations.—Superficially, the coccygeal fascia; internally, its fellow; laterally, the sacro-coccygeus lateralis; deeply, the vertebræ.
3. Sacro-coccygeus lateralis (M. sacro-coccygeus dorsalis lateralis; curvator coccygis).—This muscle lies immediately lateral to the preceding.