ARTERIES OF HUMAN LOWER EXTREMITY 77 
_ The a. ischiadica passes through the sacropudendal plexus as 
before and reaches the lateral side of the a. tibialis, which is now 
contained in the same sheath as the n. peronaeus. After its 
exit from the pelvis the artery gives origin to a few gluteral 
branches and leaves the n. tibialis with the n. cutaneus femoris 
posterior. In company with the latter nerve and the v. ischiadica, 
it passes to the posterior surface of the thigh and ends by 
dividing into several cutaneous branches. This part of the a. 
ischiadica persists as the a. glutaea inferior. 
The more distal portion of the a. ischiadica is traceable as a 
definite path through a plexus upon the posterior surface of the 
m. adductor magnus, but has lost its continuity with the a. 
poplitea. The other vessel participating in the formation of 
the plexus is the perforating branch of the a. profunda femoris. 
This plexus evidently represents the terminal anastomosis 
between the perforating arteries of the adult. It probably takes 
a large share in the formation of the perforating arteries 
themselves. 
The distal part of the anterior tibial artery has received blood 
since the time of its first appearance by way of the a. poplitea 
profunda and r. perforans cruris. An alternative path has now 
been provided by the appearance of a vessel which, passing 
around the distal border of the m. popliteus, connects the 
developing a. peronaea with a more distal part of the a. poplitea 
profunda. It is the r.communicans medius which will later become 
the definitive proximal end of the a. tibialis anterior. The 
significance of this vessel in relation to the formation of the 
adult aa. profunda and tibialis anterior will be dealt with in the 
succeeding section. 
The principal changes which have occurred in the embryonic 
arteries of the posterior crural region since the preceding stage 
of development involve the vessels which participate in the 
formation of the adult a. peronaea. These changes cons st in 
the disappearance of considerable parts of the aa. interossea and 
peronaea posterior superficialis. 
The portion of the a. interossea which formerly extended from 
the r. perforans cruris.to the distal end of the r. communicans 
