496 The Phylogeny of the Palmar Musculature 
and it would seem more satisfactory and more in harmony with the 
results of comparative anatomy to speak of only one adductor pollicis, 
regarding it, however, as consisting of a caput obliquum and a caput 
transversum. 
10. The interossei—F rom what has been said in connection with the 
flexor brevis pollicis it will be seen that I am in accord with Henle in 
his contention that there are really four palmar interossei present in the 
human hand, that to the thumb, however, being frequently unrecogniz- 
able either from its small size or on account of its incorporation with the 
oblique head of the adductor. 
Recognizing the identity in number of the dorsal and palmar interossei, 
it remains to consider their mutual relationship and their equivalents in 
the lower vertebrates. Ruge (1880) in his paper on the development of 
the deep muscles of the human foot, considered briefly the interossei of 
the hand and, by showing that these muscles resembled in their develop- 
ment their homologues in the foot, established the important point that 
the dorsal interosseil were in reality portions of the palmar musculature, 
their final dorsal position being secondary. He went, however, even 
farther than this and regarded the dorsal interossei of the foot as in all 
respects equivalent to the palmar interossei; in other words, he regarded 
them both as derivatives of the same foundamental layer, considering 
Cunningham’s assignment of them to different layers as erroneous. It 
seems to me, so far as the interossei of the hand are concerned, that the 
correct position is an intermediate one between those held by these two 
authors. I believe that the interossei really represent two fundamental 
layers but that there has been a considerable amount of union between 
the two layers to form the dorsal interossei, these muscles consisting of 
elements from the flexor brevis profundus combined with the intermeta- 
carpales. ; 
For the sake of avoiding repetition of details I would refer back to 
what has been said in the previous chapier regarding the representatives 
of these layers in the mammalia I have studied and merely state that the 
intermetacarpals, though less evident than in the opossum, are yet 
much more distinct in the human hands I studied than in either the cat 
or the mouse, as may be seen from the inspection of the adjacent figure 
(Fig. 10, im). 
I believe the significance of the mammalian interossei to be as follows: 
The flexor brevis profundus is represented by a series of paired slips for 
each digit inserted into the opposite sides of the proximal phalanges, 
except in the cases of the first and fifth digits in which one of the slips 
is inserted into the adjacent side of the second and fourth digit, respec- 
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