176 TOPOGRAPHICAL ANATOMY OF THE 



iliac, which is produced by the juuction of the external iliac and hypo- 

 gastric veins. The obturator vein may be a tributary of the hypo- 

 gastric, but it more usually joins the external iliac vein. 



A. SACRA MEDIA. — The middle sacral artery arises from the end of 

 the aorta in the angle of divergence of the two hypogastric arteries, and 

 runs for a variable distance along the mid-ventral line of the sacrum. 

 It frequently happens that this artery is entirely absent ; and, at its 

 best, the size of the vessel is inconsiderable. 



Pelvic lymph glands. — A number of scattered lymph glands 

 (lymplioglandul?e sacrales) should be found on the pelvic surface and 

 about the borders of the sacrum. A chain of glands (lymphoglandulae 

 rectales) follows the dorsal surface of the rectum ; and three or four 

 glands of small size (lymphoglandul;v anales) are related to the sphincter 

 muscle of the anus. 



Nn. lumbales : Nn. sacrales. — The time has arrived when it is 

 possible to make a general survey of the lumbar and sacral nerves. 

 These agree in number with the vertebrae of the lumbar and sacral 

 regions respectively, and leave the vertebral canal by the intervertebral 

 foramina caudal to the vertebrae with which they are in numerical 

 correspondence. Each nerve divides into dorsal and ventral branches. 

 Owing to coalescence of the transverse processes of the sacral vertebra, 

 the dorsal and ventral branches of the sacral nerves (except the last) 

 escape by the dorsal and ventral sacral foramina respectively. The 

 dorsal branches are distributed to the muscles and skin of the sacral 

 region and the root of the tail. The ventral branches of five lumbar 

 and five sacral nerves form a plexus that is simple at the beginning of 

 the lumbar region, but becomes increasingly complicated as the sacrum 

 is approached. The first lumbar nerve scarcely ever enters into the 

 plexus formation. 



It is here convenient to summarise the constitution of the nerves 

 derived from the lumbar and sacral plexuses, though many of them have 

 already been examined in detail. 



N. iliohypogastricus constitutes the ventral branch of the first 

 lumbar nerve. 



]\\ ilioingitinalis consists mainly of fibres derived from the second 

 lumbar, but may receive a small reinforcement from the third nerve. 



N. genitofemoralis (n. spermaticus externus) is the ventral branch 

 of the third lumbar, to which are generally added fibres from the second 

 and fourth nerves. 



N. cutaneus femoris lateralis is formed by fibres from the third 

 and fourth, and may receive an addition from the fifth lumbar nerve. 



