_J. Playfair McMurrich 467 
different innervations or with Gegenbaur (1889), that muscles though 
similar in all other respects but differing in their innervation are 
entirely different structures. Gegenbaur bases his conclusion on the 
assumption that the muscle is the end organ of a motor nerve, and, 
granting this and granting also that the principal nerve trunks of a 
limb are throughout exactly equivalent. his conclusion is logical. But 
there is evidence to show that the second assumption is unwarranted. 
In other words, the muscle may be regarded as the end organ of the 
nerve fibre, but that fibre need not in all cases follow the same path to 
reach its destination; in some cases it may follow the path marked by 
the ulnar trunk and in another that indicated by the median. The 
origin and termination of a nerve fibre are in all probability definite 
in their relations, but the relations of the intervening portion of the 
fibre may vary greatly. The nerve will follow in general the path of 
least resistance and this may carry it in one case into one of the larger 
trunks and in another into a different one, and we may thus have equi- 
valent muscle fibres supplied from different nerve trunks, but yet by 
equivalent nerve fibres. 
But the lines of least resistance which the main nerve trunks will 
tend to follow are to a marked extent definite, being largely associated 
in the limbs with the arrangement of the muscles in layers. Conse- 
quently, unless there be sufficient reasons to the contrary, the position 
of the main nerve stems may be taken as guides for the homology of 
certain of the muscle layers, especially in the amphibia and reptilia, in 
which there is great similarity in the arrangement of the main nerve 
trunks. It is of importance, therefore, to indicate the general arrange- 
ment of the principal nerves with reference to the muscle layers in the 
amphibia. 
The nerves of the amphibian forearm are three in number, a ramus 
profundus, a ramus superficialis ulnaris and a ramus _ superficialis 
medialis. Of these the last is confined entirely to the forearm so far 
as its muscular distribution is concerned, the other two being con- 
tinued into the hand. The ramus profundus at the wrist rests directly 
upon the carpal bones and as it is traced distally curves to lie be- 
tween the flexor brevis profundus III and the flexor brevis medius IT, 
later on breaking up into a number of branches which supply the mus- 
cular slips associated with the second and third digits and with the 
radial side of the fourth. The ramus superficialis ulnaris at its entrance 
into the hand also lies directly upon the volar surface of the carpus 
and is continued onward between the ulnar slips of the flexor brevis 
profundus and flexor brevis medius, eventually breaking up into 
