278 



FASCLE AND MUSCLES OF THE HORSE 



Acting together, they maj- 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. Smal 



bursa; may be found between the ribs and 

 tendons. 



Relations. -Superficially, the dorsal and 

 ventral serrati; deeply, the external inter- 

 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 

 ribs ventrally; consequently it has the form 

 of a three-sided prism. 



Origin. — (1) The tubera, crest, and ad- 

 jacent part of the ventral surface of the ilium; 

 (2) the first three sacral spines; (3) the lum- 

 bar and thoracic spines and the supraspinous 

 ligament. 



Insertion. — (1) The lumbar transverse 

 and articular processes; (2) the thoracic 

 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 Avith 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-iliac 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. 



Fig. 270. — Right Portion of Cross-section of 

 Back of Horse. Section is Cut Through 

 Seventh Thoracic Vertebra. 

 a, Ligamentum nuchge; b, trapezius mus- 

 cle; c, cartilage of scapula; d, latissimus dorsi; e, 

 cutaneus; /, rhomboideus thoracalis; g, ser- 

 ratus ventralis ; h, serratua dorsalis; h', lumbo- 

 dorsal fascia, which divides below into three 

 layers; i, longissimus costarum; k, levator costEe; 

 k'. interna! intercostal muscle; I, longissimus dorsi; 

 m, m' , seventh thoracic vertebra; n. head of 

 eighth thoracic vertebra; o, head of eighth rib; 

 p, seventh rib; r, intercostal artery and nerve; s, 

 skin. The fasciee are indicated by dotted lines. 

 (After EUenberger, in Leisering's Atlas.) 



