432 The Phylogeny of the Plantar Musculature 
afforded by the frequency with which, throughout the mammalia, as 
described by Cunningham, 82, the lateral head disappears, leaving the 
medial head as the sole representative of the muscle, and also by the 
fact pointed out by Ruge, 78, and which I can confirm, that the tendon 
of the flexor longus hallucis throughout the greater part of its rela- 
tionship with the muscle rests wpon and not to the medial side of the 
lateral head (Fig. 8). For if the medial head, as there is every reason 
to believe, is a portion of the flexor brevis superficialis, then a muscle 
lying dorsal to the long flexor tendon cannot be regarded as belonging 
to the same layer that it does. 
In accepting this view I differ, however, from Cruveilhier and Flem- 
ming as to the morphological significance of the lateral head. The 
medial head (Fig. 8, fbh), as just stated, is undoubtedly a portion of 
the flexor brevis superficialis, but instead of referring the lateral head 
to the adductor layer, 7%. e., to the flexor brevis medius str. profundum 
I would assign it to the flexor brevis medius str. superficiale, 7. ¢., to 
the layer from which the lumbricals are derived. This conclusion is 
based upon the comparative series shown by the lacertilia, the opossum 
and man, in all of which the same muscle is recognizable and in the 
first named group is evidently a portion of the flexor brevis medius str. 
superficiale. The muscle, then, may be regarded as the lumbrical of 
the hallux.’ 
Flemming, 87, attaches considerable importance to the nerve supply 
of the lateral head being from the lateral plantar, while that of the 
medial head is from the medial plantar. In so describing the supply 
he follows the account given in most of the continental anatomies. Cun- 
ningham, 87, however, takes exception to this, stating that not only 
in the human foot but in those of all the mammals he studied, with one 
exception, the supply of both heads was from the medial plantar and 
my own observations on one adult and two fetal human feet give the 
same result. The nerve supply, however, cannot be taken as a criterion 
in this matter, especially if the lateral head of the muscle be regarded 
as belonging to the lumbrical layer; for the lumbrical of the second 
digit is normally supplied by the medial plantar and, furthermore, 
1Tt seems probable that the corresponding medial head of the flexor brevis 
pollicis has the same significance. In an earlier paper, 03, I referred it to 
the adductor set, but the arrangement in the foot throws new light upon the 
question. Young, 79, has identified in the Rock kangaroo a muscle, distinct 
from the flexor brevis pollicis, as a pollical lumbrical, but its isolated occur- 
rence makes it questionable whether it can properly be regarded as such, 
rather than as an anomaly. 
