THE FASCI4 AND MUSCLES OF THE BACK AND LOINS 277 
First LAYER 
1. Trapezius thoracalis. 
2. Latissimus dorsi. 
Seconp LAYER 
3. Rhomboideus thoracalis. 
The foregoing are described with the other muscles which attach the thoracic 
limb to the trunk (p. 293). 
4. Serratus dorsalis anterior (s. cranialis) —This is a thin quadrilateral muscle, 
named from its serrated ventral border. It lies under cover of the rhomboideus, 
serratus ventralis, and latissimus dorsi. 
Origin.—The lumbo-dorsal fascia and dorso-scapular ligament. 
Insertion.—The lateral surfaces of the fifth or sixth 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. The muscle- 
fibers pass ventrally and backward to be attached to the ribs by seven or eight 
digitations below the lateral edge of the longissimus costarum. 
Relations.—Superficially, the rhomboideus, serratus ventralis, latissimus dorsi, 
and serratus dorsalis posterior; deeply, the longissimus dorsi, longissimus costarum, 
external intercostal muscles, and the ribs. 
Blood-supply.—Intercostal arteries. 
Nerve-supply.—Thoracic nerves. 
5. Serratus dorsalis posterior (s. caudalis)—This muscle resembles the pre- 
ceding one, which it partly covers. 
Origin.—The lumbo-dorsal fascia. 
Insertion.—The lateral 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 ventrally 
and forward and terminate in seven or eight digitations, one or two of which cover 
the posterior teeth of the anterior muscle. The aponeurosis blends with that of 
the latissimus dorsi. 
Relations.—Superficially, the latissimus dorsi and external oblique; deeply, 
the longissimus dorsi, longissimus costarum, external intercostals, serratus dorsalis 
anterior, and the ribs. 
Blood-supply.—Intercostal and lumbar arteries. 
Nerve-supply.—Thoracic nerves. 
THIRD LAYER 
6. Longissimus costarum.'—This long, segmental muscle extends across the 
series of ribs, in contact with the outer edge of the longissimus dorsi. 
Origin.—(1) The deep layer of the lumbo-dorsal fascia as far back as the third 
or fourth lumbar transverse process. (2) The anterior borders and lateral surface 
of the last fifteen ribs.? 
Insertion —The posterior borders of the ribs and the transverse processes of 
the last cervical vertebra. 
Action.—Chiefly to depress and retract the ribs and so help in expiration. 
‘This muscle is also known as the long costal, ilio-costalis, or transversalis costarum. The 
name given above was introduced by the International Commission for the Reform of Myological 
Nomenclature and appears worthy of general adoption. 
? The lumbar part of this muscle is subject to variation. It may, in quite exceptional cases, 
extend as far as the ilium, and sometimes, on the other hand, it does not arise quite as far back as 
given above. In some subjects the origin can be traced distinctly to the tips of lumbar transverse 
processes. 
