304 The Nerves and Muscles of the Leg 
which develop from common anlages. We shall therefore first take up 
the gastrocnemius-soleus group, then the deep musculature of the back 
of the leg, then the musculature innervated by the lateral plantar, and 
finally that innervated by the medial plantar nerve. The nerve supply 
of the muscles of the back of the crus is taken up after treating the em- 
bryonic development of the two sets of muscles in this region; the nerve 
supply of the plantar musculature is taken up after considering the 
differentiation ot the plantar muscles. 
1. Development of the Gastrocnemius-soleus Group. 
M. gastrocnemius.—As pointed out by Schomburg, oo, the lateral por- 
tion of the flexor plate of the leg gives rise to the gastrocnemius and 
soleus muscles. The anlage of the gastrocnemius is the more lateral and 
superficial of the two muscles and shows two incompletely separated 
heads (Plate IX, Fig. 3). These heads are connected by a fairly dense 
tissue with the anlage of the calcaneus but do not extend across the tibial 
nerve to the femur. During the latter half of the second month the heads 
of the gastrocnemius develop rapidly. In an embryo of 20 mm. (Plate 
IX, Fig. 5) the lateral head of the gastrocnemius has formed a tendinous 
attachment above the lateral condyle of the femur while the medial head 
has not yet quite reached its final destination. The nerves to the gastroc- 
nemius enter each head of the muscle soon after the anlages appear. The 
nerves may be readily distinguished in an embryo of 20 mm. (Plate IX, 
Fig. 5). 
In the adult the fibre bundles of each head of the gastrocnemius take 
an oblique and nearly parallel, though somewhat diverging, course from 
the tendons of origin to the tendon of insertion. The nerve to each head 
enters about the middle third of the superior margin of the muscle and 
its main branches take a course distally across the obliquely running fibre 
bundles, a course corresponding to the course of the nerve in the embryo. 
M. soleus—The anlage of this muscle is closely associated with that 
of the gastrocnemius. It hes beneath and projects beyond the tibial 
margin of the gastrocnemius (Plate IX, Fig. 4). It arises on the upper 
end of the fibula and distally extends into an anlage of the tendon of 
Achilles. At first it is as large as the gastrocnemius. During subsequent 
development it extends over the posterior tibial nerve to be attached to 
the tibia. This attachment is not completed in an embryo of 20 mm. 
(Plate IX, Fig. 6). The nerves for the muscle arise at an early stage 
as shown in Plate IX, Fig. 4. Their distribution in an embryo of 20 
mm. is shown in Plate IX, Fig. 6. 
