Part 28

_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.

_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.

_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.

_Origin._—The sides of the sacral spines, with the multifidus, and the transverse processes of the sacral and coccygeal vertebræ.

_Insertion._—The lateral surface of the coccygeal vertebræ, except the first four.

_Action._—Acting with its fellow, to assist the preceding muscle in elevating the tail; acting singly, to incline it to the same side.

_Structure._—This muscle appears to be a direct continuation of the multifidus dorsi. The belly is fusiform and receives reinforcing fasciculi from the transverse processes of the sacrum. This is succeeded by bundles of tendons, as many as four lying alongside of each other.

_Relations._—Superficially, the lateral sacro-iliac ligament and the coccygeal fascia; dorsally, the sacro-coccygeus superior; ventrally, the intertransversales; deeply, the vertebræ and a branch of the lateral coccygeal artery and accompanying vein and nerve.

4. =Intertransversales caudæ= (Mm. intertransversarii caudæ).—These consist of muscular bundles which lie on the lateral aspect of the tail, between the preceding muscle and the sacro-coccygeus inferior. They begin on the lateral edge of the sacrum and occupy the spaces between the transverse processes, to which they are attached. They are, however, not arranged in a strict segmental manner.

_Action._—Acting together, to fix the coccygeal vertebræ; acting singly, to assist in lateral flexion.

5. =Sacro-coccygeus inferior= (Mm. sacro-coccygei ventrales; depressor coccygis).—This muscle lies on the ventral aspect of the sacrum and coccyx. It is composed of two portions, described by Bourgelat and the German anatomists as separate muscles.

(_a_) The =outer portion= (M. coccygeus ventralis lateralis) is much the larger of the two. It arises from the outer part of the ventral surface of the sacrum, about as far forward as the third foramen, and is inserted into the transverse processes and ventral surface of the coccygeal vertebræ.

(_b_) The =inner portion= (M. sacro-coccygeus ventralis medialis) arises from the ventral surface of the sacrum internal to the preceding muscle and the first eight coccygeal vertebræ, and is inserted into the ventral surfaces of the coccygeal vertebræ.

_Action._—Acting together, to depress (flex) the tail; acting singly, to incline it laterally also.

_Structure._—The outer portion has a somewhat compressed belly, and receives bundles from the transverse processes of the coccygeal vertebræ. The inner portion is much smaller and shorter, reaching only about to the middle of the tail.

_Relations._—Ventrally, the pelvic and coccygeal fasciæ; dorsally, the sacrum, coccygeal vertebræ, and the intertransversales; laterally, the sacro-sciatic ligament, the coccygeus, and the coccygeal fascia; medially, its fellow, the recto-coccygeus, and the middle coccygeal vessels. Branches of the lateral coccygeal vessels and nerves lie between the outer division of the muscle and the intertransversales.

_Blood-supply._—Middle and lateral coccygeal arteries.

_Nerve-supply._—Coccygeal nerves.

THE MUSCLES OF THE THORAX

These consist of seven muscles or sets of muscles, which are attached to the thoracic vertebræ, to the ribs and their cartilages, and to the sternum. Functionally, they are muscles of respiration.

1. =Levatores costarum.=—These constitute a series of small muscles which occupy and overlie the upper ends of the intercostal spaces.

_Origin._—The transverse processes of the thoracic vertebræ.

_Insertion._—The external surfaces of the upper ends of the ribs posterior to the vertebral origin.

_Action._—To draw the ribs forward in inspiration.

_Structure._—Arising by tendinous fibers, each muscle passes backward and outward and expands at its insertion. Some fibers pass over one rib and are inserted on a succeeding one. At the first and last spaces the muscle cannot be distinguished from the external intercostal, of which it is in reality only a specially developed part.

_Relations._—Superficially, the longissimus dorsi; deeply, the ribs, internal intercostal muscles, and the intercostal vessels and nerves.

_Blood-supply._—Intercostal arteries.

_Nerve-supply._—Intercostal nerves.

2. =External intercostals= (Mm. intercostales externi).—Each of these occupies an intercostal space, from the levatores to the sternal extremity of the rib. They do not occupy the intercartilaginous spaces.

_Origin._—The posterior borders of the ribs.

_Insertion._—The anterior borders and external surfaces of the succeeding ribs.

_Action._—To draw the ribs forward in inspiration.

_Structure._—The fibers are directed downward and backward. There is a considerable admixture of tendinous tissue. The thickness of the muscles gradually diminishes toward the lower ends of the spaces.

_Relations._—Superficially, the serratus magnus, latissimus dorsi, serratus anticus and posticus, longissimus dorsi, transversalis costarum, rectus thoracis, deep pectorals, obliquus abdominis externus, and panniculus; deeply, the internal intercostals and (in the upper part of the spaces) the intercostal vessels and nerves.

_Blood-supply._—Intercostal and internal thoracic arteries.

_Nerve-supply._—Intercostal nerves.

3. =Internal intercostals= (Mm. intercostales interni).—These occupy the entire length of the intercostal spaces, including their interchondral portion.

_Origin._—The anterior borders of the ribs and their cartilages.

_Insertion._—The posterior borders of the preceding ribs and cartilages.

_Action._—To draw the ribs backward in expiration. It seems probable, however, that the intercartilaginous portion is inspiratory.[47]

_Structure._—The direction of the fibers is oblique downward and forward. There is a smaller amount of tendinous tissue than in the external set, and the thickness diminishes from below upward. In the upper part of the spaces fibers sometimes cross a rib in a fashion similar to the subcostals of man. A thin aponeurosis separates the internal from the external intercostal muscle in each space.