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The Nerves and Muscles of the Leg 
peroneal and tibial nerves into a common trunk may account for the 
variation in the relations of those nerves to the muscle in the adult. 
Although Gegenbaur and others have considered the piriformis to be de- 
rived from the caudo-femoral muscle of urodeles and reptiles, both com- 
parative anatomical and embryological studies speak against this view. The 
caudo-femoral muscle of these lower forms is represented in many of the 
mammals by a caudo-femoralis (W. Leche), which typically extends from 
the caudal vertebre to the lateral side of the distal half of the femur and 
runs parallel with the presemimembranosus. As in the reptiles and urodeles 
so here the muscle toward its femoral insertion lies in front of the sciatic 
nerve while the piriformis normally runs dorsal to this nerve. 
The piriformis is to be looked upon as an especially differentiated portion 
of the ilio-femoral muscle of urodeles and reptiles. In a considerable number 
of mammals it is not differentiated (some ungulates, etc.).* 
In man the piriformis is frequently fused with the gluteus medius. Its 
origin may take place from the great sciatic notch instead of from the sacrum. 
b. Nerve Variation in the Adult. 
1. Variation in the Relations of the Superior Gluteal Nerve and of the 
Nerve of the Piriformis to the Nerve Roots. 
The preceding tables, XX, XXI, indicate the frequency of certain 
modes of origin from the sacral plexus of the superior gluteal nerve and 
the nerve to the piriformis muscle and the relation of these modes of 
origin to certain types of Jumbo-sacral plexuses. While there is some 
correspondence between an anterior or a posterior form of plexus and a 
“high” or “low” mode of origin of the nerves this correspondence is 
by no means perfect. 
2. Variation in the Branches of Distribution. 
Supertor gluteal nerve-—Most frequently this nerve arises by two roots, 
one from the lumbo-sacral cord (4th-5th lumbar) and the other from 
the first sacral nerve. The trunk usually soon divides into two branches. 
The ascending branch is distributed mainly to the more dorsal part of 
the gluteus medius muscle in the middle third between its tendens. Ac- 
cording to some authors it also sends fibres to the gluteus minimus 
muscle. J have found it much more frequently confined in distribution 
to the gluteus medius muscle. The transverse branch passes across the 
external surface of the gluteus minimus muscle about midway between 
its tendons and near the lateral border of the muscle passes beneath a 
*“ According to Kohlbrugge, g7, the piriformis has a double origin, on the 
one side from the gluteal musculature, on the other from the metameric 
caudal muscles. 
