THE SriXAL NERVES. 189 



127 J/7, M?>, j\fg, and 3fio) are best described together, as 

 they are intimately associated to form the sciatic plexus [Plexvs 

 iscJiiafliciis, Flexus cnirali^). 



Within the vertebral canal the roots of these nerves form the 

 Cauda 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 sacrum or ninth ver- 

 tebra and the urostyle, and the tenth by the Canalis coccygeus in 

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

 the ventral surface of the M. ilio-coccygeuH to the pelvis, where they 

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

 usual arrangement is that the seventh and eighth nerves vmite . 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 

 off or receives the following branches.: — 



{a) Like other spinal nerves, these nerves give oRBami communi- 

 cantes (see Sympathetic System). 



[b] The M. Hio-lnjpogadricu!< (Fig-. 127 AI/]a). This arises from 

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

 nerve ; it runs oblicjuely outwards and downwards on the 3Iu-sc. 

 ilio-coccygeus and under the border of the il/. ohliquns abdoin. in- 

 ternus, where it bifurcates : — 



(1) One branch, the Ramus culanetis ahdominalis, pierces the 

 M. obliq. abdom. inter nus in the same fashion as the cutaneous 

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

 the skin of the abdomen. 



(3) The second or Ramus muscularis supplies the flat abdominal 

 muscles and the M. rectus abdominis. 



(e) The 31. cruralis (Fig. 127 Ilyb) also arises from the seventh 

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

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

 the thigh, where it lies upon the 31. ilio-jjsoas in a triangle between 

 the 31. rectus fern, anterior and the il/, adductor longus, and divides 

 into two main branches : — 



(i) Muscular twigs to the 31. ilio-jjsoas, 31. rectus feworis anticus, 

 3I3I. addnctores longus and brevis. 



(2) The Ramus cutaneiis femoris, which runs downwards in the 

 hinder wall of the Lymph-sac [Saccus iliacus), and supplies the skin 

 of the under and outer sm-faces of the thia-h. 



