LEVATORES COSTARUM MUSCLES. 373 



cygeal nerve by loops on the back of the sacrum. A few filaments are 

 distributed to the back of the sacrum and the coccyx. 



Coccygeal nerve (1 c). Its posterior primary branch issues through the 

 lower aperture of the spinal canal, and appears by the side of the coccyx. 

 It is joined by a loop from the last sacral nerve, and ends on the posterior 

 surface of the coccyx. 



* SACRAL ARTERIES. Small branches leave the spinal canal with the 

 sacral nerves ; they supply the muscular mass of the erector spinae, and 

 anastomose with branches on the back of the sacrum from the gluteal and 

 sciatic arteries. 



* Dissection. The examination of the posterior part of the wall of the 

 thorax may be made before the body is again turned. By removing, oppo- 

 site the ribs, the ilio costalis and longissimus dorsi, the small levatores cos- 

 tarum will be uncovered. The hinder part of the external intercostal 

 muscle will be denuded at the same time. 



* The LEVATORES COSTARUM are twelve small fan-shaped muscles, 

 which are connected with the hinder part of the ribs. Each, except the 

 first, arises from the apex and lower border of the transverse process of a 

 dorsal vertebra ; and is inserted, the fibres spreading out, into the upper 

 border of the rib beneath, from the tubercle to the angle. The muscles 

 increase in size from above down, and their fibres have the same direction 

 as the external intercostal layer. 



The first is fixed above to the transverse process of the last cervical 

 vertebra, and below to the outer border of the first rib. Some of the four 

 lower muscles are continued beyond one rib to that next succeeding : these 

 longer slips have been named levatores longiores costarum. 



Action. The muscles raise the hinder part of the ribs, as the name sig- 

 nifies, and the lowermost draw the bones somewhat back. 



* The external intercostal muscle is continued backwards along the ribs 

 as far as the tubercle, and is overlaid by the elevator muscle. Beneath 

 the muscle are the intercostal nerve and artery. 



Dissection. To trace the anterior and posterior primary branches of the 

 dorsal nerves to their common trunk, the elevator of the rib and the ex- 

 ternal intercostal muscle are to be cut through in one or more spaces. The 

 intercostal artery with its posterior branch is laid bare by this proceeding. 



* The dorsal nerves split in the intervertebral foramina into anterior 

 and posterior primary branches. 



* The posterior branches are directed backwards, internal to the anterior 

 costo-transverse ligament, and have been examined (p. 3G7). 



The anterior, named intercostal, is continued between the ribs to the 

 front of the chest : its anatomy is learnt in the dissection of the thorax 

 (p. 343). 



* The intercostal artery has an almost exact correspondence with the 

 dorsal nerve in its branching and distribution. 



