318 The Nerves and Muscles of the Leg 
b. Branches to adductor brevis from anterior and posterior divisions. 
2 (E) (B) (M) (B) (L) (G) 
ec. Branch to adductor brevis from posterior division. 
4 (E) (B) (M) (G) (L) 
1 (E) (B) (M) (G) (L, C) 
Ill. THE SCIATIC NERVE. 
In early embryonic life the separation between the tibial and peroneal 
nerves is well marked nearly to their origin from the sacral plexus. Near 
the plexus there intervenes between them a considerable amount of dense 
tissue (Plate II, Fig. 3) and more distally they are separated by the 
anlage of the fibula (Plates III, IV, and V). 
a. Embryonic Development. 
The peroneal nerve in avn embryo 11 mm. long (Plate III, Fig. 2) 
may be traced as far as the middle of the dorsal side of the limb-bud. 
Four fairly distinct muscle anlages are visible along its course. The 
first of these, the gluteus medius mass, represents the anlage of the 
gluteus medius and minimus, the piriformis and the tensor fascie late, 
and toward it special branches: are proceeding from the plexus. The 
second muscle mass represents the anlage of the gluteus maximus and 
the third that of the short head of the biceps. These two anlages adjoin 
one another. The fourth represents the anlage of the extensors of the 
ankle and peroneal muscles. Some differentiation is apparent between 
the anlages of the last two groups of muscles. In an embryo of 14 mm. 
(Plate IV, Fig. 2; Plate VIII, Fig. 4; Plate IX, Fig. 1) muscle differ- 
entiation has taken place in each of the anlages mentioned above and 
the anlage of the short extensor of the toes has appeared. To each muscle 
rudiment a nerve branch is given. In an embryo of 20 mm. (Plate V, 
Fig. 2; Plate VIII, Fig. 5; Plate IX, Fig. 2) muscle differentiation is 
more marked and the branches to each muscle resemble somewhat those 
of the adult. 
The tibial nerve in an embryo of 11 mm. (Plate III, Fig. 1) extends 
to the middle of the plantar side of the leg. Along its course several 
muscle anlages may be seen. Of these the first is that of the obturator 
internus, the second that of the quadratus femoris, the third that of the 
hamstring muscles, the fourth that of the gastrocnemius—soleus group, 
and the fifth that of the deep muscles of the back of the leg. In an 
embryo of 14 mm. (Plate IV, Fig. 1; Plate VIII, Fig. 1;, Plate IX, 
Figs. 3 and 4) individual muscles have appeared in each of the anlages 
mentioned and a muscle mass has appeared in the foot. In the leg the 
