THE SPINAL XF.EV 189 



- - M-. MS, .1/9, and J/io) are best described together, as 

 they are intimately associated to form the sciatic plexus (Plow* 

 iscJiiaflicHS, Plexv.% cn/rati*). 



Within the vertebral canal the roots of these nerves form the 

 Cav.Ja equina; the seventh nerve leaves the canal between the 

 seventh and eighth vertebrae, the eighth nerve between the eighth 

 and ninth vertebrae, the ninth between the sac-nun or ninth ver- 

 tebra and the urostyle, and the tenth by the Canali* coccygeii* in 

 the urostyle ; from these points the four nerves run obliquely on 

 the ventral surface of the 31. ilio-cpccygev* to the pelvis, where they 

 form the sciatic plexus. Although subject to some variation, the 

 usual arrangement is that the seventh and eighth nerves unite to 

 form a trunk, which then receives the ninth nerve ; the large nerve 

 so formed is the sciatic nerve : a branch of the tenth usually then 

 joins either the sciatic plexus or the sciatic nerve; the plexus gives 

 lijff or receives the following branches : 



(a) Like other spinal nerves, these nerves give off Kami connnvni- 

 cantes (see Sympathetic System). 



() The 31. ilio-Afpogastrictu (Fig. 127 31 ~ a}. This ari-e* from 

 the seventh spinal nerve, before its union with the eighth spinal 

 nerve; it runs obliquely outwards and downwards on the 3Ic---. 

 ilio-coccygeit and under the border of the 31. oWifjiius aljtlom. ln- 

 Iffnii*, where it bifurcates : 



(1) One branch, the Ranni* cutaneus aldominalix, pierces the 

 , f iq. aldviii. i ut trnu* in the same fashion as the cutaneous 



branches of the fourth, fifth, and sixth spinal nerves ; it supplies 

 the skin of the abdomen. 



(2) The second or Ramus mmculari* supplies the flat abdominal 

 muscles and the 31. reefi'* alilominis. 



fa) The 31. cnrra Kg. 127 31 ~b] also arises from the seventh 

 spinal nerve at or just beyond its point of union with the eighth 

 spinal nerve. It accompanies the A. entrails on the pelvic wall to 

 the thigh, where it lies upon the 31. Ulo-j^oiu in a triangle between 

 the 3f. re?! it* fr,,t. anterior and the 3f. ail >1 net or longu*, and divides 

 into two main branches : 



(i) ^luscular twigs to the 31. ilio-p*oa*, 3f. rectus ftmori* a/ificitx, 

 313 f. addnetorc* lotiiiz and 



(2) The Ramm cvfanev* fernon*. which runs downwards in the 

 hinder wall of the Lymph--; x . and supplies the skin 



of the under and outer surfaces of the thisrh. 



