ARTERIES OF HUMAN LOWER EXTREMITY 67 
The course of the axial artery is somewhat less straight than 
in the preceding stage. This is due chiefly to a sharp convexity 
directed toward the growing extremity of the a.femoralis. The 
convexity is surmounted by a short sprout indicating the point 
at which the femoral is later to unite with the axial artery. 
Although the axial artery is straighter in the preceding stage 
than at the stage of 12 mm., an irregularity is noticeable at the 
stage of 8.5 mm. in a similar situation. It seems to represent 
an earlier indication of the convexity which is so pronounced at 
the present stage of development. 
Distal to the knee the axial artery lies in the narrow interval 
between the tibia and fibula. The interosseous membrane and 
individual muscles are still unrecognizable, but, although the 
artery bulges toward the extensor region, its distal course 
clearly indicates that it does not leave the flexor aspect of the 
leg. 
At what appears to be the proximal end of the very short 
tibiofibular interspace a second bend occurs in the axial artery. 
This bend is so pronounced as to practically amount to the bud 
of the vessel which is shortly to grow into the extensor aspect of 
the leg from this point. 
The dorsal and plantar retia of the foot are richer than in the 
preceding stage. The connection between the axial artery and 
the dorsal rete is now reduced to a single vessel of large size, the 
r. perforans tarsi. 
The a. iliaca externa has bifurcated into the a. epigastrica 
inferior and the a. femoralis.* The latter is contrasted sharply 
from the a. iliac externa by the thinness of its walls, which con- 
sist of endothelium only. Its structure resembles very closely 
that of the femoral vein which accompanies it. The femoral 
artery runs parallel with and upon the medial side of the n. 
saphenus. Its extremity, now about half way along the femur, 
is bifurcated into a lateral and a medial branch. The former is 
short and will later join the axial artery; it may be called the 
6 That the a. epigastrica inferior is an independent branch of the a. iliaca 
externa, which considerably antedates it and the a. femoralis in development, 
has already been pointed out by the writer (Senior, ’17). 
