400 NERVOUS SYSTEM. 
3. N. ischiadicus (Fig. 162, 4; Fig. 163, a).—The great 
sciatic nerve, the largest nerve in the body, arises from the 
lumbosacral cord (formed chiefly by the sixth and seventh 
lumbar nerves), the first sacral nerve and a small branch from 
the second sacral. It passes caudodorsad and leaves the pelvis 
by passing across the great sciatic notch, between M. pyri- 
formis and M. gemellus superior. It passes across the tendon 
of M. obturator internus (Fig. 163, 9) near the insertion, and 
here sends caudad a large muscular branch (4) beneath the 
biceps. This branch innervates the biceps, semitendinous (17) 
and semimembranosus (is). Other twigs in this region pass to 
the quadratus femoris (10) and tenuissimus. The great sciatic 
nerve (a) now passes across the quadratus femoris (10), adductor 
femoris (15), and semimembranosus (16), lying beneath the 
biceps femoris. It thus enters the popliteal space and ap- 
proaches the popliteal artery and vein. Here it gives off one 
or two small muscular branches to the distal part of the 
biceps, and sends a slender branch, N. suralis (c), along the 
medial surface of the biceps to the lateral surface of the lateral 
head of M. gastrocnemius (20). The sural nerve (¢) becomes 
cutaneous at the distal (ventral) border of the biceps, and 
divides three or four centimeters proximad of the ankle into 
two branches. One passes over the tendon of Achilles (27) to 
the proximal end of the calcaneus and ramifies in this region. 
The other passes onto the lateral surface of the foot, and sup- 
plies the integument in this region over the tarsus and part of 
the metatarsus. 
The great sciatic nerve now divides in the popliteal space 
into the. peroneus communis (¢@) and the tibialis (¢). 
a. N. peroneus communis (@).—This is the more lateral 
of the two divisions of the great sciatic nerve. It passes along 
the medial surface of the biceps to the lateral surface of the 
lateral head of M. gastrocnemius (20), where it extends to a 
point just distad of the head of the fibula. Here it passes 
beneath that part of the gastrocnemius which has origin on the 
fascia of the shank, passes between the soleus and peroneus 
longus (21), then between the peroneus longus and peroneus 
tertius. It gives off a number of small muscular branches, 
