346 The Nerves and Muscles of the Leg 
peroneus brevis. In four instances the nerves of supply of the peroneus 
longus arose from the n. peroneus superficialis (by one branch in one 
instance). In five instances a proximal branch (in one instance, two) 
arose from the n. peroneus and a more distal branch from the n. peroneus 
superficialis. 
In some of the instances above cited the nerves of supply subdivided 
before entering the muscle. 
IX. MUSCULATURE OF THE EXTENSOR SIDE OF THE CRUS AND FOOT. 
a. Embryonic Development. 
1. General Features. 
Tn an embryo of 11 mm. (Plate III, Fig. 2) the peroneal nerve ex- 
tends over the dorsal surface of the limb-bud and ends in a mass of 
shghtly differentiated myogenous tissue, the anlage of the extensor 
muscles of the leg and foot. This anlage is more or less fused with the 
anlage of the peroneal muscles. 
In an embryo 14 mm. long (Plate IX, Fig. 1) the peroneal nerve has 
given rise to the nn. peroneus superficialis and profundus. The n. per- 
oneus profundus may be traced to the region between the bases of the 
first two metatarsals. Above and on each side of it may be distinguished 
muscle anlages representing the extensor muscles of the leg and foot. 
To these muscle anlages nerves are given as shown in the figure. The 
tendons of the extensor digitorum and extensor hallucis proprius are rep- 
resented by a sheet of tissue in which the segmentation is just beginning. 
The conditions here described correspond well with those pictured by 
Schomburg, 00, except in a few minor details to which attention is called 
in considering the development of the individual muscles. 
In an embryo of 20 mm. (Plate IX, Fig. 2) the individual muscles 
and their tendons, as well as the nerves distributed to them, indicate 
clearly relations corresponding in many features with those characteristic 
of the adult. 
2. Individual Muscles. 
Tibialis anterior—The anlage of this muscle becomes distinct from 
the general dorsal myogenous sheet of the limb-bud during the sixth 
week. In an embryo of 14 mm. (Plate IX, Fig. 1) the muscle anlage 
is most distinctly differentiated in the region where the two nerves are 
extending into it. From here it may be followed distally into a broad 
tendon which fades out over the region of the first cuneiform and the 
base of the first metatarsal. In an embryo of 20 mm. (Plate IX, Fig. 2) 
