THE FASCI.E AND MUSCLES OF THE BACK AND LOINS 235 



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.^ 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-supph/. — Occipital artery. 



Nerve-supply. — Dorsal branch of the first cervical nerve. 



The Fascia 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 ofT the 

 longissimus. It is attached medially to the supraspinous ligament and the spinous 

 processes of the vertebrce; 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 ex- 

 ternus abdominis, the transversus abdominis, and the retractor costae. 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 lamellae which intersect the scapular part of the serratus and are at- 

 tached with it to the scapula. Three lamellae 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 vertebrae; 

 it gives attachment to the complexus. The middle one dips in lietween the longis- 

 simus 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 descriloed with the other nuiscles which attach the thoracic 

 limb to the trunk (p. 251). 



4. Serratus anticus (M. serratus dorsalisinspiratorius). — This is a thin quadri- 

 lateral muscle, named from its toothed ventral border. It lies beneath the rhom- 

 boideus, 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 ins]iiration. 



1 This seems due to pressure produced by pathological changes in the supra-atloid bursa, 

 which are frequently extensive in dissecting-room subjects. 



