390 The Nerves and Muscles of the Leg 
of the dorsum of the foot. At the end of the sixth week, as pointed out 
by Schomburg, the muscle mass is not differentiated into special parts 
(Plate IX, Fig. 1), but toward the end of the second month the bellies 
of which it is composed and their tendons begin to stand out distinctly 
(Plate IX, Fig. 2). The differentiation of the terminal tendons begins 
on the fibular side and extends toward the tibial. The nerve to this 
muscle mass arises at an early stage from the n. peroneus profundus and 
extends across its deep surface. In the adult this nerve extends across 
the component muscle bundles about midway between their tendons of 
origin and insertion. 
_ Extreme variability is shown in the form of this muscle in the adult. 
It may be absent or be reduced to two or three bundles or there may be 
an unusual development of the muscle and the differentiation into bellies 
corresponding to the digital tendons. The m. eat. hallucts brevis is the 
most frequently isolated of these bellies. 
The extensor digitorum brevis doubtless represents the remains of an 
intrinsic dorsal pedal musculature relatively better developed in urodeles 
and reptiles than in most mammals. In most urodeles (Ribbing, 06) and 
reptiles (Gadow, 82) the extensor tendons of the toes arise from these pedal 
muscles and the “extensor digitorum longus” tendons are inserted into the 
bases of certain of the metatarsals. The great variation in the development 
of the extensor digitorum brevis in man is well known. It seems to be rela- 
tively better developed in the embryo than in the adult. 
b. Nerve Distribution. 
The relations of the nerves supplying the muscles under consideration 
to the spinal nerves cannot be clearly made out by dissection. It is prob- 
able, however, that the nerves suppled to the more tibially situated 
muscles contain the greater number of the fibres springing from the 4th 
lumbar nerve, and the nerves passing to muscles situated most to the 
fibular side contain the greater number of fibres from the 1st sacral 
nerve. 
Variation in the Branches of Distribution Arising from the N. 
Peroneus Profundus. 
The nerve to the extensor digitorum brevis seems to be constant in 
its general relations, although the height at which it springs from the 
main trunk varies greatly. 
The nerves to the remaining muscles show considerable variation 
owing to the fact that the nerves to a given muscle may arise as suc- 
cessive branches from the main nerve trunk or they may be combined 
