Charles R. Bardeen 367 
muscle may be made out on the surface of the aponeurosis of the long 
flexor muscles above the region of the middle cuneiform bone. Differ- 
entiation is just beginning so that no distinct muscle fibres may be made 
out. A small nerve may be traced into its medial margin. The tendons 
have not begun to develop. Soon after this stage the muscle undergoes 
rapid development. Proximally it extends to the tuber calcanei, distally 
it sends forth tendons to the toes. 
In the adult the chief variations are those marked by a reduction of 
the muscle, especially that portion belonging to the fifth toe. The muscle 
is supplied by a nerve which enters the medial margin. 
M. abductor hallucis—This muscle is not distinctly visible in embryo 
CXLIV (length 14 mm.). It can be distinguished in embryo XXII 
(length 20 mm.), although differentiation is not here well marked. 
(Plate IX, Fig. 6). The muscle arises on the medial edge of the plantar 
surface of the foot over the navicular, first cuneiform, and the base 
of the 1st inetatarsal bones and at a considerable distance from the tuber 
mA 
calcanei. It arises in close association with the m. flexor hallucis brevis. 
With the torsion of the foot which carries the caleaneus in a medial di- 
rection the anlage of the abductor extends proximally to be attached to 
the tuber calcanei. 
In the adult a branch from the medial plantar nerve usually enters 
near the middle of the lateral border of the muscle. The relation of the 
nerve to the muscle anlage in embryo XXII is shown in the figure. 
Flexor hallucis brevis—Like the other muscles of this group this 
muscle is not distinguishable in embryo CXLIV, length 14 mm. Even 
in embryo XXII, length 20 mm. (Plate IX, Fig. 6), it is only beginning 
to appear. The cells of the anlage are closely packed together. To the 
anlage a nerve branch is given. I find the anlage somewhat more medially 
placed on the base of the first metatarsal than that shown by Schom- 
burg, oo. The anlage is incompletely divisible into two portions, a 
medial and a lateral. During further development the lateral belly ap- 
proaches the adductor hallucis. The medial belly in embryo XXII is 
associated with the abductor hallucis, although according to Schomburg 
it is brought into association with this muscle later than the lateral head 
is brought into association with the adductor hallucis. 
In the adult the nerve enters between the two bellies of the muscle and 
spreads out into branches which pass between the constituent muscle 
bundles. It is only rarely that the lateral head of the muscle is supplied 
by the lateral plantar nerve. 
