278 FASCLHZ AND MUSCLES OF THE HORSE 
Acting together, they may assist in extending the 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 
ventro-laterally. From these are detached two sets of tendons. The superficial 
tendons spring from the lateral 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 inter- 
costal spaces to its origin on the anterior border or lateral surface of a rib. Small 
bursee may be found between the ribs and 
tendons. 
Relations.—Superficially, the dorsal and 
ventral serrati; deeply, the external imter- 
costals and the ribs. The lumbar origin is 
covered by the longissimus dorsi. The deep 
cervical and dorsal vessels cross the surface of 
the muscle at the first and second intercostal 
spaces respectively, and branches of the in- 
tercostal vessels and nerves emerge between 
it and the longissimus dorsi; here a fascial 
layer dips in between the two. 
Blood-supply.—Intercostal arteries. 
Nerve-supply.—Dorsal branches of the 
thoracic nerves. 
7. Longissimus dorsi.'—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 trans- 
verse processes and the upper ends of the 
Fic. 270.—RicHt PorTION OF CROSS-SECTION OF 
Back or Horse. Secrion is Cur THroucH 
SeventH Tuoracic VERTEBRA. ribs ventrally; consequently it has the form 
a, Ligamentum nuch»; 6, trapezius mus- of a three-sided prism. 
cle; c, cartilage of scapula; d, latissimus dorsi; e, Origin = 1) The tubera. crest. and ad- 
cutaneus; f, rhomboideus thoracalis; g,  ser- 
ratus ventralis; hk, serratus dorsalis; h’, lumbo- jacent part of the ventral surface of the ilium; 
dorsal fascia, which divides below into three (2) the first three sacral spines; (3) the lum- 
layers; 7, longissimus costarum; k, levator coste; | 1 t} unin, 2AGE: Se itl ees = 
k’, internal intercostal muscle; J, longissimus dorsi; pall and thoracic spines and the supraspious 
m, m’, seventh thoracic vertebra; 7», head of ligament. 
eighth thoracic vertebra; 0, head of eighth rib; Insertion.—(1) The lumbar transverse 
p, seventh rib; 7, intercostal artery and nerve; s, ; F = 
skin. The fascie are indicated by dotted lines. and articular processes ; (2) the thoracic 
(After Ellenberger, in Leisering’s Atlas.) transverse processes ; (3) the spinous and 
transverse processes of the last four cer- 
vical vertebrae; (4) the lateral 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-iliae ligaments, 
and is attached to the crest and sacral angle of the ilium and the first and second 
sacral spines; it furnishes origin to the lumbar portion of the middle gluteus. In 
1 The muscle as here described includes the longissimus dorsi et cervicis and the spinalis and 
semispinalis components, as the separation of these is largely artificial in the horse. 
