486 The Phylogeny of the Palmar Musculature 
adductor layer: (1) an adductor pollicis which arises from the fascia 
covering the first metacarpal and is inserted into the ulnar side of the 
proximal phalanx of the pollex (Fig. 9, ad p), (2) an adductor indicis 
(ad) and (3) an adductor minimi digiti (ad m), both of which arise from 
the fascia covering the bases of the third and fourth metacarpals and are 
inserted into the proximal phalanges of their respective digits. No 
separation of the adductor minimi digiti into two portions, as is the 
case in the cat, occurred. 
Of the stratum profundum of the middle flexor which occurs in the 
reptilia I have found no representative in the mammalian hand if 
the identification of the adductor minimi digiti of the opossum with a 
part of the stratum medium be correct. It is to be noted, however, 
that superficial to the adductor there is clearly to be seen in sections 
a very thin layer of muscle tissue, separated from the surface of the 
adductor by a narrow but quite distinct layer of areolar tissue. Its 
fibres are directed less obliquely than those of the adductor and, becom- 
ing tendinous, it fades out in the fascia beneath the profundus tendon 
of the fourth digit. It is possible that this muscle really represents 
a portion of the stratum medium and that the adductor represents the 
stratum profundum. I have been able, however, to find no corre- 
sponding muscle toward the radial side of the hand in the opossum 
and it seems hardly possible that the adductor minimi digiti repre- 
sents a different layer for the adductor pollicis when their general 
similarity and their origin from a common median raphe are consid- 
ered. In the higher mammals: studied I have found nothing which 
corresponds to this muscle. 
But instead of regarding it as part of the stratum medium another 
interpretation is possible for it, and that is that it represents in a 
diminished condition the more superficial radial slip of the portion 
of the median flexor which passes to the fifth digit in the Iguana 
(Fig. 4, fbm?). Its relations to the remaining portions of the stratum 
medium are essentially the same as those of the reptilian muscle and 
there seems to be no obvious reasons for not regarding it as identical 
with the latter. 
I conclude, therefore, that the deep stratum of the reptilian flexor 
brevis medius is unrepresented as a distinct stratum in the mammalian 
hand. 
The flexor digitorum brevis profundus and the intermetacarpales.— 
We come now to the epineural muscles of the mammalian hand, those 
which correspond to the flexores breves profundi and intermetacar- 
pales of the lower vertebrates and to the intermediate and deep layers 
