THE NERVOUS SYSTEM 177 



and then enters the M. flexor primordiaHs communis. It may- 

 terminate within this muscle or it may pass right through and fuse 

 with the N. fibularis (as figured, PL XII). In the latter case a branch 

 passes from it to supply the muscle (f.p.). As already mentioned 

 an anastomosing branch may pass from the mesial side of the sciatic 

 nerve in this region to join the N. cut. fem. vent. 



At some point between the hip and the knee the sciatic nerve 

 splits to allow the sciatic artery to pass through it, but the actual 

 extent of this split is subject to great variation. It may be quite 

 short, when it occurs just proximal to the knee, or it may be of 

 moderate length, as in the figure (PI. XII). On the other hand, one 

 case has been observed in which two approximately equal nerves, 

 representing the sciatic, passed down the thigh parallel with one 

 another, and with the sciatic artery between them. One of these 

 nerves was derived from N. spin. 17 and the other from N. spin. 16. 

 They fused at the knee. This case, therefore, offers an explanation of 

 the phenomenon of an artery penetrating an apparently solid nerve, 

 and at the same time explains the variation in length of the split in 

 different specimens, since this is merely a measure of the degree of 

 fusion between the sixteenth and seventeenth nerves. 



The whole sciatic nerve leaves the thigh dorsal to the M. flex, fib., 

 and enters the leg between the insertion of the M. pub. isch. tib. 

 and the origin of the M. flex. prim. comm. Here it divides — if it has 

 not already done so just proximal to the knee — and the divisions 

 correspond very closely to the branches of the N. brachialis in the 

 fore-arm. They have, therefore, been given analogous names in 

 spite of the fact that the term 'N. fibularis' has been applied by some 

 authors to an entirely different nerve — ^viz. N. peroneus. Thus the 

 two main divisions of the sciatic nerve distal to the knee are here 

 called N. fibularis and N. interosseus, while the mesial branch of the 

 N. fibularis is termed R. medianus since it corresponds with the 

 similarly named branch of N. ulnaris in the fore-arm. 



N. fibularis (n.f.) (mihi). 



External popliteal ...... Humphry (187 1). 



R. lateralis n. ischiadici . .... Hoffmann (1873-8). 



This nerve is not to be confused with Hoffmann's 'fibularis', 

 which is the dorsal or extensor nerve and the equivalent of the nerve 

 here described under the name 'peroneus'. As stated above, the 

 close similarity between the nerve now under consideration and the 

 N. ulnaris in the fore-arm seems to the writer sufficient justification 

 for giving it an analogous name. 



4038 N 



