THE DEEP MUSCLES OF THE BACK 399 



mass varies in size and structure at different parts of the vertebral column. In 

 the sacral region it is narrow and pointed, and at its origin chiefly tendinous in 

 structure. In the lumbar region it is larger, and forms a thick fleshy mass which, 

 on being followed upward, is subdivided into three columns; these gradually 

 diminish in size as they ascend to be inserted into the vertebrae and ribs. 



The Sacrospinalis arises from the anterior surface of a broad and thick tendon, 

 which is attached to the medial crest of the sacrum, to the spinous processes of 

 the lumbar and the eleventh and twelfth thoracic vertebrae, and the supraspinal 

 ligament, to the back part of the inner lip of the iliac crests and to the lateral 

 crests of the sacrum, where it blends with the sacrotuberous and posterior sacro- 

 iliac ligaments. Some of its fibers are continuous with the fibers of origin of the 

 Glutseus maximus. The muscular fibers form a large fleshy mass which splits, 

 in the upper lumbar region into three columns, viz., a lateral, the Iliocostalis, an 

 intermediate, the Longissimus, and a medial, the Spinalis. Each of these consists 

 from below upward, of three parts, as follows: 



Lateral Column. Intermediate Column. Medial Column. 

 Iliocostalis. Longissimus. Spinalis. 



(a) I. lumborum. (a) L. dorsi. (a) S. dorsi. 



(b) I. dorsi. (6) L. cervicis. (b) S. cervicis. 



(c) I. cervicis. (c) L. capitis. (c) S. capitis. 



The Iliocostalis lumborum (Iliocostalis muscle; Sacrolumbalis muscle) is inserted, 

 by six or seven flattened tendons, into the inferior borders of the angles of the lower 

 six or seven ribs. 



The Iliocostalis dorsi (Musculus accessorius) arises by flattened tendons from 

 the upper borders of the angles of the lower six ribs medial to the tendons of 

 insertion of the Iliocostalis lumborum; these become muscular, and are inserted 

 into the upper borders of the angles of the upper six ribs and into the back of the 

 transverse process of the seventh cervical vertebra. 



The Iliocostalis cervicis (Cervicalis ascendens) arises from the angles of the third, 

 fourth, fifth, and sixth ribs, and is inserted into the posterior tubercles of the trans- 

 verse processes of the fourth, fifth, and sixth cervical vertebrae. 



The Longissimus dorsi is the intermediate and largest of the continuations of 

 the Sacrospinalis. In the lumbar region, where it is as yet blended with the Ilio- 

 costalis lumborum, some of its fibers are attached to the whole length of the pos- 

 terior surfaces of the transverse processes and the accessory processes of the lumbar 

 vertebrae, and to the anterior layer of the lumbodorsal fascia. In the thoracic 

 region it is inserted, by rounded tendons,' into the tips of the transverse processes 

 of all the thoracic vertebrae, and by fleshy processes into the lower nine or ten ribs 

 between their tubercles and angles. 



The Longissimus cervicis (Transversalis cervicis), situated medial to the Longis- 

 simus dorsi, arises by long thin tendons from the summits of the transverse pro- 

 cesses of the upper four or five thoracic vertebrae, and is inserted by similar tendons 

 into the posterior tubercles of the transverse processes of the cervical vertebrae 

 from the second to the sixth inclusive. 



The Longissimus capitis ( Trachelomastoid muscle] lies medial to the Longissimus 

 cervicis, between it and the Semispinalis capitis. It arises by tendons from the 

 transverse processes of the upper four or five thoracic vertebrae, and the artic- 

 ular processes of the lower three or four cervical vertebrae, and is inserted into the 

 posterior margin of the mastoid process, beneath the Splenius capitis and Sterno- 

 cleidomastoideus. It is almost always crossed by a tendinous intersection near 

 its insertion. 



The Spinalis dorsi, the medial continuation of the Sacrospinalis, is scarcely 

 separable as a distinct muscle. It is situated at the medial side of the Longissimus 



