J. Playfair McMurrich 487 
of Cunningham’s scheme. It must be pointed out that while the two 
layers taken together correspond with Cunningham’s two layers, indi- 
vidually the layers of the two sets are quite different. I shall not dis- 
cuss this point here, however, but reserve it for the concluding chapter, 
and, in the meantime, would merely point out that the flexores breves 
profundi and intermetacarpales are so intimately associated in the 
mammalha that it is not feasible to discuss them separately. 
In the opossum and other marsupials Young, Brooks and Cunning- 
ham have recognized in their intermediate layer a series of flexor 
muscles corresponding to the various digits, each muscle being com- 
posed of two shps which pass to the ulnar and radial sides of a proximal 
phalanx. Such a condition recalls, it is true, the amphibian arrange- 
ment of the profundus layer, but it must be remembered that the 
reptilian arrangement is quite different, one of the two slips of certain 
muscles transferring its insertion to an adjacent digit, and I believe 
that in the mammalian hand there has been a similar transference of 
some of the slips. 
In the opossum I find on the ulnar side of the hand two muscles 
belonging to the flexor brevis profundus, one (Fig. 6, fbp,) arising 
from the fascia enclosing the long tendon to the fifth digit and being 
perforated close to its origin by the deep branch of the ulnar nerve, 
and the other (Figs. 6 and 7, fbp’) taking its origin from the base of 
the fifth metacarpal and lying to the radial side of the first. The ulnar 
muscle passes directly distally and inserts into the base of the proximal 
phalanx of the minimus, while the other passes obliquely across the 
interval between the fifth and fourth metacarpals and unites with the 
intermetacarpal of that interval and with the ulnar slip of the flexor 
brevis profundus of the fourth digit to be inserted into the proximal 
phalanx of that digit. The ulnar muscle I take to be the radial slip 
of the portion of the intermediate layer passing to the minimus as 
described by Young and Brooks, but they seem to have overlooked 
the radial muscle, although it is evidently identical with the slip which 
Cunningham has described in Thylacinus and Phalangista (1882) as a 
palmar slip entering into the formation of the fourth dorsal inter- 
osseous. The ulnar minimal slip which these authors recognize in the 
fifth digit belongs, I believe, to the abductor mass and therefore to an 
entirely different layer from the radial slip. 
The muscles referred by the same authors to portions of the inter- 
mediate layer passing to the annulus medius and index are identical 
with those which I take to be the flexores breves profundi of the 
same digits (Fig. 7, fbp*, fbp’ and fbp*); they arise in pairs from the. 
