498 The Phylogeny of the Palmar Musculature 
importance of obtaining a correct idea as to the fundamental significance 
of the mammalian hand musculature is sufficiently great to interest all 
students of vertebrate morphology in the questions discussed. I have, 
therefore, endeavored to state in a series of propositions the main con- 
clusions which I have expressed both in this paper and in a preceding 
one (1903) so far as it concerns the hand musculature. 
1. In the urodele amphibia the volar hand muscles are arranged in 
four distinct layers which may be named the flexor brevis superficialis, 
flexor brevis medius, flexor brevis profundus and intermetacarpales. 
2. In the lacertiha the number of these layers is increased to seven 
by the subdivision of the flexor brevis superficialis into a stratum super- 
ficiale and a stratum profundum, and of the flexor brevis medius into 
a stratum superficiale, a stratum medium and a stratum profundum. 
3. In the mammalha the number of clearly recognizable layers is five, 
the str. profundum fi. brevis superficialis and the str. profundum fl. 
brevis medii of the lacertilia being apparently wanting. 
4+. In the mammalia the greater portion of the flexor brevis super- 
ficialis has degenerated to form the palmar portions of the tendons of 
the flexor sublimis digitorum, marginal portions of it persisting, how- 
ever, to form the abductor and opponens pollicis, the abductor and 
opponens quinti digiti, the flexor brevis pollicis, the flexor brevis quinti 
digiti, the palmaris brevis and, in some cases, a palmaris brevis radialis. 
5. The palmar portions of the flexor profundus digitorum are derived 
from a layer of fascia which, in the lower forms, intervenes between the 
flexor brevis superficialis and the flexor brevis medius, the str. super- 
ficiale of the fl. brevis medius arising in these lower forms from this 
fascia. 
6. The str. superficiale of the fl. brevis medius gives rise to the mam- 
malian lumbricales. 
7. The str. profundum of the fl. brevis medius gives rise to the mam- 
malian adductors. 
8. The flexor brevis profundus in the mammalia consists of paired 
slips for each digit. Certain of these slips remain distinct and form the 
palmar interossei; the remainder unite with the intermetacarpales to 
form the dorsal interossei. 
9. The term flexor brevis as applied to muscles of individual digits 
(pollex and minimus) is appropriately limited to muscles derived from 
the flexor brevis superficialis. 
10. The flexor brevis pollicis as defined by Albinus is composed of ele- 
ments derived from both the flexor brevis superficialis and the str. pro- 
fundum of the flexor brevis medius. Only the outer head which is de- 
