236 FASCI.E AND MUSCLES OF THE HORSE 



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, ex- 

 ternal intercostal muscles, and the ribs. 



Blood-supply. — Intercostal arteries. 



Nerve-supply. — Thoracic nerves. 



5. Serratus posticus (M. serratus dorsalis exspiratorius) . — This muscle re- 

 sembles 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 latissi- 

 mus dorsi. 



Relations. — Superficially, the latissimus dorsi and external oblique; deeply, 

 the longissimus dorsi, transversalis costarum, external intercostals, serratus anti- 

 cus, 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 verte- 

 bras 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 vertebrae. 



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 ril). The deep tendons are detached 

 from the dorsal part of the deep face of the nuiscle. Each passes backward across 

 one or two intercostal spaces to its origin on the anterior border or outer surface 

 of a rib. Small bursse may be found between the ribs and tendons. 



Relations. — Superficially, the dorsal serrati and the complexus; deeply, the ex- 

 ternal intercostals and the ribs. The lumbar origin is covered l)y the longissimus. 

 The superior (deep) cervical and dorsal vessels cross the surface of the muscle at 

 the first and s(H'ond 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. 



' Tlie lumbar part of this muscle is subject to variation. It may, in quite exceptional cases, 

 extend as far as the ilium. 



