400 NERl^OUS SYSTEM. 



3. N. ischiadicus (Fig. 162, /r, Fig. r63, a).— The great 

 sciatic nerve, the largest nerve in the body, arises from the 

 himbosacral 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 (^) beneath the 

 biceps. This branch innervates the biceps, semitendinosus (17) 

 and semimembranosus (ic). 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 (ig), 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 (r), along the 

 medial surface of the biceps to the lateral surface of the lateral 

 head of M. gastrocnemius (20). The sural nerve (c) 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 (d) and the tibialis (r). 



a. N. peroneus communis (d). — 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,. 



