478 The Phylogeny of the Palmar Musculature 
logeny of the forearm flexors I showed that the flexor brevis superficialis 
of the reptilia and amphibia was represented in the mammalia partly 
by the terminal portions of the tendons of the flexor sublimis digitorum. 
Since, however, this muscle sends no tendon to the pollex, we may well 
expect to find some special representative of the superficial flexor in the 
radial side of the mammalian hand, and, furthermore, it is not unlikely 
that portions of the ulnar border of the sheet may have persisted and 
even slips of its more median portion. 
That this likelihood is reality can, I think, be readily perceived by 
the study of a series of mammalian hands. In the opossum there is 
superficially in the ulnar part of the hand, in addition to the sublimis 
Fic. 6. Transverse section through the wrist of the opossum. abp, abductor 
pollicis; 6°, flexor brevis minimidigiti; fbp, and fbp},, radial and ulnar slips of the 
flexor brevis profundus; ev, tlexor carpi radialis; /?~*, lumbricales; m, median 
nerve; ma, os Magnum; p' °, tendons of the flexor profundus digitorum; pl, pal- 
maris longus; s, radial sesamoid; s?~*, tendons of the flexor sublimis digitorum; 
td, trapezoid; tm, trapezium; w, unciform; ws and wp, superficial and deep branches 
of the ulnar nerve; x, remnant of the flexor brevis superticialis. 
tendon, a well marked mass of muscle tissue, consisting of several more 
or less distinct slips arising partly from the sheath of the long tendons 
and partly from the hook of the unciform. A portion of this muscle 
tissue passes superficial to the superficial branch of the ulnar nerve 
to be lost in the fascia covering the abductor minimi digiti, and may 
well be regarded as representing the palmaris brevis of other mammals. 
The greater bulk of the mass (Fig. 6, fb"), however, passes between the 
superficial (ws) and deep (wp) branches of the nerve to be inserted into 
the outer part of the cartilage which covers the volar surface of the 
