434 The Phylogeny of the Plantar Musculature 
the first and second intermetatarsals forming the first and second dorsal 
interossel, which insert into the proximal phalanx of that digit. The 
medial slip of the third digit forms the first plantar interosseous, while 
the lateral slip unites with the third intermetatarsal to form the third 
dorsal interosseous; the medial slip of the fourth digit forms the second 
plantar interosseous, while its lateral slip unites with the fourth inter- 
metatarsal to form the fourth dorsal interosseous. 
In the fifth digit both slips of the flexor brevis profundus retain their 
separate individualities, the medial one forming the third plantar in- 
terosseous, while the lateral one is the muscle known as the flexor 
brevis quinty digit. 
I have found nothing in my preparations that I could regard as a 
hallucal flexor profundus, although on comparative grounds such 
muscles should be expected. There is a probability, judging from what 
occurs in the opossum, that the first dorsal interosseous contains an 
element representing a hallucal flexor profundus, just as is the case in 
the hand, but of this I have not been able to obtain definite evidence. 
The so-called interosseous primus volaris of Wood, 67, does not appear 
to belong to the interosseous set of muscles; its position towards the 
medial surface of the hallux and its relations with the abductor are 
opposed to such an assignment of it. It seems rather to be a slip of 
the flexor brevis hallucis which has retained its primary origin from 
the first cuneiform, instead of shifting to the plantar aponeuroses with 
the rest of the muscle. So too the portion of the oblique head of the 
adductor hallucis, which Henle, 71, regards as the equivalent of the 
interosseus primus volaris of the hand, is rather to be regarded as a 
portion of the flexor brevis medius str. profundum. 
In the preparations I have studied there is no distinct opponens 
hallucis, nor is the opponens quintt digiti represented as a distinct 
muscle. I am unable to determine the significance of the former 
muscle, whether it be a derivative of the adductor or the flexor brevis 
hallucis, but viewing the possibilities as they appear in my prepara- 
tions I am inclined to look to the adductor for its origin (cf. Brooks, 
87). In a fetus of 9 cm. I find that a portion of the flexor brevis 
quinti digiti inserts upon the upper part of the fifth metatarsal, a fact 
which seems to point to the derivation of the opponens quinti digiti from 
the flexor brevis. In this opinion I am in accord with Ruge, 78. It 
is certainly a very different structure from the so-called opponens quinti 
digiti of the cat (see p. 426). 
Many difficulties are encountered in the working out of the detailed 
a 
