190 The Phylogeny of the Forearm Flexors 
The separation of the N. brachialis inferior longus into its forearm 
branches has, in the intermediate forms between the reptilia (or amphi- 
bia) and mammalia, receded up the arm until in the mammalia it oceurs 
practically at the brachial plexus. During the recession a very con- 
siderable change in the relative position of the R. profundus has oe- 
curred, since it has left its deep situation and come forwarl to join the 
greater part of the R. superficialis radialis, forming a stem, the median, 
which hes ventral to the deeper muscles and has on the whole a radial 
position. 
That portion of the original profundus, however, which supplies 
forearm muscles remains more or less distinct from the median and 
forms the anterior interosseous nerve which passes to its destination 
anterior to the pronator quadratus. 
The ramus superficialis medialis, which, even in the lower forms, 
sphts into numerous branches soon after its entrance into the forearm, 
probably associates itself partly with the R. profundus to form the 
median and partly jois the R. superficialis ulnaris to form the ulnar, 
and it.is in the relative amounts of it which enters into the composition 
of each of these nerves that variation occurs in the mammalia. 
Bearing in mind, then, that we probably have in the mammalhan 
anterior interosseous nerve the representative of the portion of the R. 
profundus which is supplied to the forearm, the portion of that nerve 
destined for the hand being included in the main stem of the median, 
and considering also the topographic relations of the deep portions of 
the flexor communis digitorum suppled by it, it seems that we are justi- 
fied in identifying these portions of the communis with the portion of 
the reptilian palmaris profundus which is supplied by the R. profundus ; 
in other words the radialis and ulnaris portions of the flexor communis 
are together almost equivalent to portion I of the palmaris profundus. 
But not entirely so, since as a rule the ulnar portion of the ulnaris also 
receives some twigs from the ulnar nerve, and the portions of the muscle 
so supplied probably represent another portion of the deep muscles of 
the lower forms, but exactly which, it is difficult to-state with certainty. 
There seem to be two possibilities worthy of consideration; either (1) 
the twigs for the ulnar nerve, which enter the muscle, represent a por- 
tion of the R. ulnaris, in which case it is necessary to turn to the am- 
phibia to find a homologue for the muscle fibres in the ulno-carpalis, 
or (2) the twigs represent a portion of the R. superficialis medialis 
which has associated itself with the R. ulnaris, and in this case the 
muscle fibres would represent either the second or third portion or both 
these portions of the palmaris profundus. I am not prepared to say 
