THE NERVOUS SYSTEM 175 



2 and 3 — while the posterior branch is similarly distributed to 

 digits 3 and 4, forming A^A^. digitales dorsales 3 and 4. The posterior 

 nerve also receives an anastomosis from the R. accessorius n. dors, 

 ped. fib. 



N. ischiadicus ventralis (n.i.v.). Appleton (1928). 



Arises from a 16—17 anastomosis as a nerve of moderate size. It 

 passes ventral to the M. caudali-femoralis, between it and the M. 

 pubo-ischio-femoralis externus. It first gives one or two small 

 branches to the M. ischio-femoralis (is.fm.) and then turns antero- 

 ventrally between the M. caud. fem. and M. pub. isch. fem. ext., 

 giving off a large twig to supply the MM. pubo-ischio-tibialis 

 (p.i.t.), pubo-ischio-femoralis externus (p.i.f.e.), and ischio-flexorius 

 (pars propria) (is.f.). It then turns more anteriorly and gives off a 

 large musculo-cutaneous branch. The cutaneous elements go to the 

 skin covering the antero-ventral surface of the leg and form part of 

 the A^. cutaneus femoris ventralis (n.c.fm.v.). They emerge between 

 the M. pub. isch. tib. and the M. pub. tib., while the muscular 

 twigs supply the M. pubo-tibialis (p.t.). The nerve also gives at, 

 or near, this point a small branch to the M. pubo-femoralis (p.fm.). 

 The remaining portion of the N. isch. vent, now becomes the TV. 

 cutaneus femoris ventralis (n.c.fm.v.), and passes along the thigh to 

 the knee between the muscles pubo-tibialis and pubo-ischio-tibialis. 

 At the knee it becomes superficial and passes right along the tibial 

 border of the fore-leg beneath the skin. It is continued to the ex- 

 tremity of the first digit as the N. digitalis ventralis i (n.d.v.i). At 

 the base of the digit it sends a branch mesialwards over the skin of 

 the sole of the foot, from which a twig may arise to anastomose with 

 a terminal branch of the N. interosseus, thus completing a plantar 

 nerve loop corresponding with the palmar loop of the hand. It is 

 very doubtful whether this anastomosis is a constant feature, since, 

 when present, it is always exceedingly fine and difficult to dissect 

 without breaking. The N. cut. fem. vent, may receive a branch from 

 the sciatic nerve in the popliteal region, but this also does not seem 

 to be constant. 



Seventeenth spinal nerve (ventral branch) (n.sp. 1 7). 



N. spinalis 1 7 emerges from the sacral vertebra and passes almost 

 transversely to the posterior border of the acetabulum, where it 

 joins the sixteenth nerve, and, after anastomosing with it, continues 

 into the leg as the sciatic nerve, from which arise the ventral or 

 flexor nerves of the leg. 



