420 The Phylogeny of the Plantar Musculature 
and corresponds to Ruge’s contrahentes. The second and third layers he 
dorsal to the nerve and together correspond to Ruge’s layer of interossei ; 
the more superficial muscles Cunningham termed the intermediate layer 
of flexores breves and the deeper one the dorsal layer of abductors. 
In the urodeles and lacertilia I have shown that three (or four) layers 
are distinctly recognizable dorsal to the long flexor tendons or their 
homologue, and for this reason Cunningham’s arrangement of the deep 
muscles is preferable to that of Ruge. His dorsal layer of abductors is 
in part equivalent to what have been described in the preceding pages of 
this paper as the intermetatarsales; his intermediate layer is similarly 
equivalent in general to my flexores breves profundi; while his plantar 
layer of adductors corresponds to my flexor brevis medius str. profundum. 
But by the exclusion of the superficial layers—indeed of everything su- 
perficial to and connected with the long flexor tendons, both Cunningham 
and Ruge fall into error in the assignment to their “ intrinsic” or deep 
muscles of certain structures which are derivatives of the superficial 
layer. There are in the foot two superficial layers of muscles which are 
just as properly termed intrinsic as are the adductors and interossei, and 
in what follows I shall recognize the same layers as have been described 
in the foot of the lower forms, giving special names to certain of the 
marginal muscles when this seems desirable. 
The flexor brevis superficialis. In the mammalia studied the flexor 
brevis superficialis was represented by both the strata described in the 
lacertila. In the superficial stratum of the lacertilia there was a distinct 
tendency for the muscle slip for the hallux to separate from the rest of 
the layer; this becomes more pronounced in the mammals, in which the 
slip becomes practically an independent muscle. Furthermore, we must 
include in this layer certain marginal muscles of the foot, which may be 
termed abductors. 
Considering first the main portion of the muscle, it arises in the 
opossum from the plantar surface of the strong tendon of the flexor 
fibularis cruris, a short distance above the ankle joint, and partly also 
from the tendon of the flexor tibialis at about opposite the ankle joint, 
and is distributed by four slips (Figs. 5 and 6, fbss), to the second, third, 
fourth, and fifth digits, each slip dividing into two tendons, between 
which passes a tendon of the long flexor. In the cat and mouse the fibers 
arise from the aponeurosis into which the long flexors insert. The dif- 
ference, however, is more apparent than real, since, as has already been 
pointed out, MeMurrich, 04, both the aponeurosis and the tendons of the 
long flexors represent portions of an original plantar aponeurosis. 
