Comparative morphological remarks on the origin of the posterior femoral cutaneous nerve

Abstract

The origin and course of the posterior femoral cutaneous nerve were observed macroscopically in 38 Japanese adult cadavers which were dissected in the University of Hokkaido, Faculty of Medicine during the years 1971/72 and the results obtained were compared with those from some other mammals (rat, rabbit, dog and cat) and a number of bibliographical findings on the other animals. On the basis of the archetype of the pudendal plexus, the site of origin of the posterior femoral cutaneous nerve was divided into seven portions as follows: the sciatic nerve or inferior gluteal nerve (I) and its originating roots (RI), the bigeminal nerve (B) and its originating roots (RB), the part of junction of I and B (CIB), the pudendal nerve (P) and its originating roots (RP). According to the arising mode, the posterior femoral cutaneous nerve was calssified into seven types: Type A (the sciatic nerve type); the nerve arises from I and RI (horse, rat, bird, frog and salamander). Type B (the sciatic transitional type); the nerve arises from I, RI, CIB, RB and B (MAN AND MONKEY). Type C (the bigeminal nerve type); the nerve arises from CIB, RB and B (gorilla, chimpanzee, orangutan, cat and sphenodon). Type D (the pudendal transitional type); the nerve arises from CIB, RB, B, RP and P (dog). Type E (the pudendal nerve type); the nerve arises from RP and P (pig, cattle and rabbit). Type F (the mixed type); a mixture of A to E types. These various patterns in the posterior femoral cutaneous nerve may be explained by the comparative anatomical explanation on the limb medial rotation given in Braus' text-book of Anatomy (Bd. I, S. 273). From these descriptions it is reasonable to presume that the main trunk of the posterior femoral cutaneous nerve of the tetrapod below the Aves arises from the sciatic nerve and is analogous to the gluteal branches of mammals, with its main stem still retained in the pudendal nerve. If the cutaneous area supplied by the posterior femoral cutaneous nerve expands to the lateral border of the buttock in company with the lower limb medial rotation, the part between this area and that supplied by the pudendal nerve is enlarged. At first, these expanded areas are probably supplied by the branches of the pudendal nerve, which gradually become independent to become the main stem of the posterior femoral cutaneous nerve in mammals. This nerve seems, therefore, to be primarily a division of the pedendal nerve, and so in man has various types of arising patterns, A to E, in accordance with the scheme in the phylogeny. Those hypothetical changes are observed in the human sacral plexus, from which the cutaneous nerve arises with a fan-shaped overlapping.