482 The Phylogeny of the Palmar Musculature 
longus, the other from the radial side of the sheath enclosing the long 
tendons. The two slips converge to a delicate tendon which was lost 
in the dermal tissue over the metacarpal bone of the pollex, although 
in the adult it has been traced to the proximal phalanx, the muscle 
being that known as the abductor brevis pollicis. 
It is probable that the muscle which has been termed the flexor 
brevis pollicis (cf. Reighard and Jennings, 1901, p. 184, Fig. 89a)’ 
is also a derivative of the superficial flexor. It arises from the os 
magnum and the volar surface of the fascia covering the tendon of 
the flexor carpi radialis and is inserted into the radial side of the prox- 
imal phalanx of the thumb. In sections it seems to lie in a plane 
entirely dorsal to the long profundus tendons, but it must be remem- 
bered that it represents a marginal portion of the flexor superficialis 
which wraps around the margin of the hand to a certain extent. Its 
origin overlaps somewhat the origin of the adductor pollicis and this 
relation combined with the fact that it is supplied by a branch of the 
median nerve leads me to regard it as a portion of the flexor super- 
ficialis rather than to incline to the other possibility that it is a por- 
tion of the flexor brevis profundus. 
The mouse possesses both a palmaris brevis and a flexor brevis minimi 
digiti (Fig. 9, fb’) as well as an abductor minimi digiti (ab°), all having 
essentially the same relations as the corresponding muscles in the opossum. 
The abductor pollicis is represented by two slips which arise from the 
crescentic sesamoid cartilage of the wrist joint and are inserted into 
the radial side of the first pollical phalanx and a well developed flexor 
brevis pollicis (Fig. 9, fb’) is also present, arising from the volar surface 
of the anterior annular ligament as far medially as the line of the long 
profundus tendon for the index and passing to the outer side of the 
proximal phalanx of the thumb. In addition to these two muscles 
a third occurs in close association with the flexor brevis, consisting of 
a thin band lying immediately beneath the pad on the radial side of 
the hand, and having the same relations to it as the palmaris brevis has 
to the ulnar pad. It seems to owe its existence to the separation of 
some fibres from the flexor brevis pollicis and might be termed the 
palmaris brevis radialis. 
I have not been able to distinguish any structures in the mammalian 
hand which could with certainty be looked upon as representatives 
of a stratum profundum of the flexor brevis superficialis such as occurs 
1T refer to this work alone of those that have been written on the anatomy of the 
cat, since it does not seem necessary to enter into an extended discussion of the 
myology of this form and this is the latest extended work on the subject. 
