J. Playfair MceMurrich, 489 
of their metacarpals instead of from their bases, and those of the index 
haying assumed a volar position so as to lie, as has already been pointed 
out, in the same plane as the adductors. The thumb possesses two 
muscles of the flexor profundus set, an ulnar one which passes across the 
intermetacarpal space (fbp') to be inserted into the index and a radial 
one which is inserted into the proximal phalanx of its own digit. The 
first and fourth intermetacarpals I was not able to distinguish, they being 
probably intimately united with the muscles with which they insert. 
Those of the second and third imtermetacarpal spaces (im and im’) are 
fairly distinct, but less so than in the opossum, and are more volar in 
position. 
In the mouse (Fig. §) the general] arrangement is practically the same 
as in the two forms already described and need not be discussed in detail. 
The intermetacarpals are even less distinguishable from the slips of the 
flexor brevis profundus with which they are associated than are those 
of the cat. 
IV. THe Mutscixs or tHt Human HAnp. 
In considering the musculature of the human hand it will probably 
eonduce to clearness if, instead of discussing it from the standpoint of 
the fundamental layers, the various muscles be taken up in sequence. 
1. The palmaris brevis occupies a characteristic position with refer- 
ence to the superficial branch of the ulnar nerve, being the only muscle 
which Jies volar to it. Throughout the series of forms studied this 
relation is constant and affords a simple clew for the identification of 
the muscle, which, be it noted, is in the opossum closely associated with 
the flexor brevis minimi digiti. It is evidently a portion of the flexor 
brevis superficialis. | 
2. The flexor brevis quinti digitt.—The significance of this muscle has 
been discussed by Brooks (1886), with the conclusion that it really 
belongs to the adductor set, 7. e. to the flexor brevis medius. Brooks has 
evidently been deeply influenced by Cunningham’s views as to the 
layers of the palmar musculature, and his assignment of the muscle 
under discussion to the adductor layer may be taken as meaning that 
it is an element of a hyponeural layer, 7. e. of a layer volar to the deep 
branch of the ulnar nerve. I have shown that in addition to the palmar 
(adductor) layer, which was the only hyponeural layer recognized by 
Cunningham, there are really two others to which that term may be 
applied, namely the lumbrical layer and the flexor brevis superficialis 
layer, and it seems highly probable that the flexor brevis quinti digiti 
