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The Nerves and Muscles of the Leg 
X. MUSCULATURE OF THE PLANTAR SIDE OF THE CRUS AND FOOT. 
a. General Features. 
In an embryo 11 mm. long (Plate ITI, Fig. 1) the tibial nerve divides 
below the knee inte two branches. Of these that on the tibial side rep- 
resents the medial plantar, that on the fibular side the lateral plantar 
nerve. The lateral plantar branch descends to the tarsus, the medial 
plantar nerve not so far distally. Near the knee a mass of slightly 
differentiated tissue lying superficial to the nerve represents the 
gastrocnemius-soleus group of muscles. Beneath the nerves beyond this 
region a mass of slightly differentiated tissue represents probably the 
anlage of the deep muscles of the calf and possibly of some of the muscu- 
lature of the sole of the foot. 
In an embryo 14 mm. long (Plate IX, Figs. 3 and 4) the muscles of 
the plantar side of the leg are so far differentiated that the individual 
muscles can be fairly clearly made out. In the drawing for the sake 
of definiteness the outlines of these muscles are made diagrammatically 
sharp but the main relations shown are true to the conditions found in 
the embryo. Two groups of muscles may be distinguished, a superficial 
lateral group composed of the gastrocnemius, soleus, and plantaris; and 
a deep more medially placed group consisting of the flexor hallucis 
longus, flexor digitorum longus, the popliteus, and the tibialis posterior. 
The gastrocnemius group is connected by a mass of tissue with the 
blastema of the calcaneus. The two long flexor muscles are connected 
by condensed tissue with a flat aponeurotic “ foot-plate” from which 
tendinous processes extend to the blastema of the metatarsals and toes. 
The medial and lateral plantar nerves extend independently from the 
region of the knee to the foot. Near where they arise there is a plexi- 
form arrangement of the fibres of the tibial nerve and from the back of 
this plexus arise the nerves to the deep muscles of the back of the leg 
and to the deep surface of the soleus muscle. The nerves to the gastroc- 
nemius-soleus group, with the exception just mentioned, arise from the 
plantar surface of the tibial nerve proximal to where this changes its 
course from the thigh into the leg. In the foot the medial plantar nerve 
spreads out superficial to the pedal aponeurosis while the lateral plantar 
nerve crosses medially beneath it. Along the course of the medial plantar 
“In the article by Bardeen and Lewis, o1, the two divisions of the tibial 
nerve are represented combined into a single trunk too far distally. 
