194 The Phylogeny of the Forearm Flexors 
nator consists in its possession of a coronoid head in addition to the 
condylar one, the median nerve passing into the forearm between the two 
heads. This condition, so far as | am aware, occurs only in man and in 
the anthropoid apes, and in these forms it is associated with a marked 
reduction in the size of the pronator quadratus. There seems to be no 
doubt but that Macalister (1868) was right in regarding the deep head 
as something quite distinct from the pronator teres proper, and I believe 
we may go further than Macalister when he says in his earher paper 
that it is to be regarded as “the germ of a superior transverse muscle, 
the upper equivalent and co-ordinate of the pronator quadratus below.” 
In its highest degree of development in the mammalia this latter muscle 
occupies the entire length of the forearm, and in Perameles and some 
species of Halmaturus, in the dog and the hyena, its proximal portions 
are united with the pronator teres (Leche). In man and the anthro- 
poids, as Macalister points out in his later paper (1869), we seem to have 
other instances of a similar fusion, in association with which there has 
been, however, a degeneration of a considerable portion of the quadratus, 
only its proximal and distal portions persisting. 
In the case of the flexor digitorum communis the modifications are 
much more complicated. The most striking peculiarity of the human 
flexor is its separation into a large flexor sublimis seu perforatus and a 
flexor profundus seu perforans, and, furthermore, the separation of the 
profundus into the profundus of anthropotomy and the flexor longus 
pollicis. 
That the occurrence of a flexor longus pollicis is due to a differentiation 
of a portion of the profundus, to be more precise of a portion or all of the 
portio radials, seems beyond question. It is a muscle which has not in- 
frequently been described as absent in the lower forms, or in other cases, 
its absence has been accounted for by a fusion with the profundus. To 
my mind neither of these expressions fits the case; the latter one implies 
that it is an independent typical constituent of the mammalian flexors 
which in certain cases has disappeared by fusion with the neighboring 
muscle, while the former imples that it is unrepresented. The occur- 
rence of the muscle in a comparatively small number of the mammalia, 
¢. g. in certain carnivores, Hylobates and man, indicates by no means 
indistinctly its secondary nature, and it certainly seems improbable that 
it could appear sporadically, as it does, without having some representa- 
tive in the arms of forms nearly allied to those which possess it. If it be 
a separated portion of the profundus, then it has a representative through- 
out the entire mammalian series, probably even in forms which lack a 
pollex, for the relation of the profundus is not primarily to the individ- 
