THE NERVOUS SYSTEM 173 



close correspondence between the course of the N. dors. ped. tib. 

 in the leg and that of the N. dors. man. rad. in the fore-arm is obvious. 



Sixteenth spinal nerve {ventral branch) (n.sp. 1 6). 



This nerve emerges from the last trunk vertebra and passes 

 somewhat obliquely postero-laterally across the dorsal pelvic region 

 to the posterior border of the acetabulum, and there splits into two 

 more or less equal parts. The anterior portion, which however may 

 be much smaller than the other, turns anteriorly to join a branch from 

 the fifteenth spinal nerve and form the N. femoralis as described 

 above, while the posterior branch enters the limb close to the 

 seventeenth nerve immediately behind the hip-joint. It anastomoses 

 closely with N. spinalis 17, but nevertheless its main continuation 

 in the thigh is the N. extensorius (n.e.) (cf. N. spin. 3, in brachial 

 plexus). The N. ischiadicus ventralis (n.i.v.) also receives a con- 

 siderable proportion of its fibres from the sixteenth spinal nerve. 



N. extensorius (n.e.). 



N. fibularis .... Hoffmann (1873-8); Appleton (1923-8). 

 N. peroneus ....... Sieglbaur (1904). 



After its formation from NN. spin. 16 and 17 the extensor 

 nerve passes dorsal to the M. iliofemoralis, and gives a twig 

 to it, and then divides into two rami which pass along the thigh 

 parallel with one another immediately ventral to the MM. iliofibu- 

 laris and extensor iliotibialis. They supply these muscles (il.f. and 

 e.il.t.), and give off one or two cutaneous branches which probably 

 represent the N. cutaneus surae lateralis (n.c.s.l.), while they also send 

 a fine twig to the knee-capsule. The two rami reunite proximal to the 

 knee to form a single nerve, or in some cases the nerve remains single 

 throughout. At the knee-capsule it becomes : 



A^. peroneus (n.pn.). 



This nerve leaves the thigh dorsal to the M. iliofib., and, execut- 

 ing a half-spiral turn, enters the leg, passing ventral to the M. 

 femoro-fibularis between this muscle and the fibula. During its 

 passage under the skin, superficial to the origin of the extensor cruris 

 et tarsi fibularis, it gives off" one or two muscular branches which 

 enter this muscle (e.c.t.f.) on its post-axial side. The nerve passes 

 to the interosseal space of the tibia and fibula, and there divides 

 into two principal rami which correspond to the NN. dors. man. 

 inter, and ulnaris in the fore-arm. They have accordingly been 

 named by Sieglbaur N. dorsalis pedis intermedius (n.d.p.i.) and A^. 



