J. Playfair MeMurrich 481 
by Young and Brooks, while the second may be termed the flexor brevis 
pollicis, although it corresponds only to the radial head of that muscle 
as recognized by the authors named. I shall consider the reasons for 
this difference in connection with the third portion of the thenar mass. 
I would recognize, then, in the opossum the following representatives 
of the flexor brevis superficialis: (1) the abductor pollicis, (2) the flexor 
brevis pollicis, (3) the tendons of the flexor sublimis digitorum with 
their associated muscle slips, (4) the flexor brevis minimi digiti, (5) 
the abductor minimi digiti and (6) the palmaris brevis. 
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Fic. 8. Transverse section through the hand of a cat embryo of 7 ecm. ad?, ad- 
ductor indicis; ad.m and ad.m!, adductores minimi; ab, abductor minimi digiti; 
Sbp, and fbp,,, radial and ulnar slips of the flexores breves profundi; im, intermeta- 
carpales; 7, lumbricales; m, median nerve; p, tendons of the flexor profundus 
digitorum; pl, tendons of the palmaris longus (i.e. prolongations of the palmar 
fascia) s, tendons of the flexor sublimis digitorum; wp and ws, deep and superficial 
branches of the ulnar nerve. 
In the cat and the mouse there is a somewhat greater distinctness 
of the various muscles belonging to this layer, together with a certain 
amount of reduction, seen especially in the cat. Thus there appears 
to be no representative in that form of the flexor brevis minimi digiti 
and the abductor minimi digiti (Fig. 8, ab’) is much reduced in size as 
is also the abductor pollicis. This last muscle is represented by two 
slips arising one in the dermal tissue over the tendon of the palmaris 
