THE DISSECTION OF THE BACK 169 



the head, and it is traversed by the occipital artery, which in this part of 

 its course gives off its descending branch (O.T. arteria princeps cervicis), 

 and its meningeal branch. 



The Third Layer of Muscles. Under this head are included a series 

 of muscular strands which stretch with a greater or less degree of continuity 

 along the entire length of the dorsal aspect of the vertebral column. In the 

 lumbar region they constitute a bulky fleshy mass which may be considered 

 the main starting-point. This mass is the nmsculus sacro-spinalis which 

 has the following origins : (i) from the spines of all the lumbar vertebrae ; 



(2) from the supraspinous ligaments which bind the lumbar spines together ; 



(3) from the dorsum of the sacrum and from the posterior sacro-iliac liga- 

 ment ; (4) from the posterior fifth of the iliac crest ; (5) from the deep 

 surface of the posterior layer of the lumbo-dorsal fascia. In great part the 

 superficial surface of this muscular mass is covered by and is adherent to 

 the posterior layer of the lumbo-dorsal fascia. 



Superiorly the sacro-lumbalis divides into three columns. The lateral 

 column first separates from the general mass, and to it the name of ilio- 

 costalis is given ; the middle column is termed the longissimus^ and the 

 medial column, which becomes quite distinct only as the upper part of the 

 dorsal region is approached, is called the spinalis. The semispinalis muscle 

 is also included in the third layer. 



The Ilio-costalis is a column of muscular bundles which extends from 

 the lumbar to the cervical region. It is separable into three segments 

 known, from below upwards, as the ilio-costalis lumborum, the ilio-costalis 

 dorsi, and the ilio-costalis cervicis. 



Ilio-costalis Ltiinboriim. This muscle and the longissimus dorsi become 

 distinct at the level of the last rib, and the interval between them is marked 

 by the exits of the lateral divisions of the posterior branches of lower 

 thoracic nerves. 



The muscle ends above in a series of six or seven slender tendons, which 

 are inserted into the angles or the corresponding parts of the lower six or 

 seven ribs. 



The Ilio-costalis Dorsi (O. T. Musculus Accessorius) arises by six or seven 

 slender tendons from the angles of the lower ribs, on the medial sides of the 

 tendons of insertion of the ilio-costalis lumborum, and it is inserted by a 

 series of similar tendons into the angles of the upper six ribs and to the 

 transverse process of the seventh cervical vertebra. 



The Ilio-costalis Cervicis (O.T. Cervicalis Ascendens}. This highest 

 segment of the ilio-costalis arises, on the medial side of the ilio-costalis 

 dorsi, by four slips which spring from the third, fourth, fifth, and sixth ribs ; 

 it is inserted into the transverse processes of the fourth, fifth, and sixth 

 cervical vertebrae. 



To display the ilio-costalis properly, the dissector should first evert the 

 lowest segment, and then in turn the middle and upper segments, but 

 whilst doing this he must take care to preserve the lateral divisions of the 

 posterior branches of the spinal nerves. 



The Longissimus is the middle and largest of the three muscle columns. 

 It extends upwards, through the thoracic and cervical regions, to the head, 

 and it also is separable into three segments : longissimus dorsi, longissimus 

 cervicis, and longissimus capitis. The interval between the longissimus 

 and the spinalis is frequently difficult to define, but if the fascia is carefully 

 cleaned from the lateral to the medial border of the longissimus in the 

 upper thoracic region, the separation will become apparent, and after it 

 has been found the attachments of the longissimus must be defined. 



Longissimus Dorsi. The dorsal part of the longissimus possesses two 

 rows of slips of insertion : a medial row of tendinous slips which are 



