THE NERVOUS SYSTEM 171 



sixteen and seventeen.^ It is noteworthy that, as compared with the 

 fore-Hmb, the N. femorahs — from the fifteenth and sixteenth nerves 

 — enters the Hmb anterior to its articulation with the pelvic girdle, 

 while in the fore-limb no nerve passes into the arm in front of the 

 glenoid cavity. With this exception there is a marked similarity 

 between the two plexuses. For example, the N. obturatorius is 

 strongly reminiscent of the N. supracoracoideus, or again, the 

 extensor or dorsal nerves of the limb arise from the plexus anterior 

 to the flexor or ventral nerves in both limbs. The comparison 

 amounts almost to identity in the distal parts of the limb — a feature 

 which is of course correlated with the close similarity of the muscula- 

 ture of the fore-arm and hand with that of the leg and foot. 



The anastomosis with the fourteenth nerve is slight and involves 

 only branches of the N. ileohypogastricus. 



Fifteenth spinal nerve {ventral branch) (n.sp. 1 5). 



This nerve emerges from the penultimate trunk vertebra and 

 passes obliquely postero-laterally towards the limb. Its first branch 

 is an anterior one to the muscles of the lateral and ventral body-wall, 

 the N, ileohypogastricus (n.il.hg.). The actual point at which this 

 branch leaves the main nerve varies considerably in different speci- 

 mens, since it may be either nearer to, or more distal from the verte- 

 bral column than shown in Plate XII, which is about the average 

 position. At what may be termed the groin, i.e. in the anterior 

 angle between the limb and the body, the remainder of the fifteenth 

 spinal nerve divides into three. The first of these branches is the N. 

 cutaneus femoris medialis (n.c.fm.m.) (Hoffmann) supplying the skin 

 over the antero-dorsal region of the thigh. Hoffmann describes a 

 branch to the M. ileo-extensorius from this nerve, but it has not been 

 possible to confirm its presence, and it probably represents an indivi- 

 dual variation. The remainder of the nerve has been called A^. cruralis 

 anterior by Hoffmann ; it gives off' the following two branches. 



N. obturatorius (n.o.) passes ventrally through the M. pubo-ischio- 

 femoralis-internus, supplying it with many twigs in doing so (p.i.f.i.), 

 and finally penetrates the obturator foramen to innervate the anterior 

 portion of the M. pubo-ischio-femoralis externus. The other branch 

 fuses with the anterior branch of N. spinalis 1 6, to form : 



N.femoralis (n.fm.). Osawa (1902); Sieglbaur (1904). 



Anterior crural nerve ...... Humphry (1871). 



N. cruralis ....... Hoffmann (1873-8). 



' Sieglbaur gives the sixteenth, seventeenth, and eighteenth for the crural plexus, but 

 he probably regards N. hypoglossus as N. spinalis 2. 



