Part 69

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.

Insertion.—The posterior borders of the ribs and the transverse processes of the last two or three cervical vertebræ.