J. Playfair MeMurrich 493 
into the proximal phalanx of the thumb, and is the radial counterpart 
of the palmar interosseus of the fifth digit, as I hope to show later on. 
The interosseus primus volaris is frequently indistinguishable from 
the deeper portions of the flexor brevis pollicis (sens. lat.), being pos- 
sibly incorporated with it, although from the fact that in a human em- 
bryo of 6 em. which I examined the slip (Fig. 10, fbp:.) was exceedingly 
small and showed evident indications of degeneration | am inclined to 
believe that the failure to distinguish it may frequently be due quite as 
much to its great reduction as to its fusion with the adjacent adductor. 
But if we suppose that it does frequently become incorporated in that 
niuscle or at all events is included in the flexor brevis pollicis as limited 
Fig. 10. Transverse section through the hand of a human embryo of 6cm. abm, 
abductor minimi digiti; ad.p, caput obliquum and ad.p!, caput transversum of the 
adductor pollicis; fb', flexor brevis pollicis; f°, flexor brevis quinti digiti; fbp, 
and fbp,,, radial and ulnar stips of the flexores breves profundi; im, intermetacar- 
pales; 7, lumbricales; m, median nerve; op®, opponens quinti digiti; p, tendons of 
the flexor profundus digitorum; s, tendons of the flexor sublimis digitorum; ws, 
superficial branches of the ulnar nerve. 
by those who adhere to Albinus’ conception of the muscle, then the 
flexor in this sense is really composed of elements from three different 
layers, namely, the sunerficialis, the deeper stratum of the medius and 
the profundus. 
G. The abductor pollicis—The abductor pollicis does not seem to exist 
as a distinct muscle in the lower vertebrates I have studied; the mam- 
malian muscle has appeared within the limits of that phylum and I 
believe, contrary to the opinion of Cunningham, that it is derived from 
the flexor superficialis. The a priort reasons which indicated an origin 
of the abductor quinti digiti from this layer holds also in the case of the 
