THE CIRCULATORY SYSTEM 183 



artery known as the common iliac is the first formed of the 

 posterior limb, and that it arises as a segmental vessel of the 

 aorta. 



The artery which, in the embryo Mammal, including Man, 

 runs into the developing posterior limb bud, does not directly 

 become the arteria femoralis of the adult. It accompanies the 

 ischiadic [or crural] nerve in its distribution ; on the posterior 

 side of the limb it runs down to the bend of the knee, and from 

 this point is continued into the upper part of the thigh. This 

 artery should be called the ischiadic [or crural] as it corresponds 

 with the vessel of the same name in most Birds, and with the 

 principal vessel of the hind-limb in Eeptiles and Amphibians. 



" The femoral artery develops l^Jgr as a branch of the iliac. 

 At first it spreads only over the inner or ventral portion of the 

 thigh ; it, however, soon grows rapidly in a distal direction, along 

 the inner surface of the cartilaginous femur, to the bend of the knee, 

 where it unites with the ischiadic artery. The femoral artery 

 thus formed rapidly increases in size, while that section of the 

 ischiadic related to the upper leg degenerates. It is thus 

 that the definitive condition is attained ; and but a short vestige 

 of the arteria ischiadica persists in the adult, as the " ischiadic " 

 or " inferior gluteal " (Hochstetter). Mechanical causes may have 

 perhaps brought about this change in the principal artery of the 

 hind-limb in the ancestors of Mammals, but we have no clear 

 knowledge on the subject. 



In no other part of the body are the variations in the arteries 

 so frequent as in the fore-limb, especially in the hand. The 

 arteries of the foot present numerous variations, and, in correla- 

 tion with the variations of the skeleton and musculature, 

 some of these may be classed as progressive and others as 

 retrogressive. 



Where a supracondyloid process of the humerus exists (cf. 

 ante, p. 78) the brachial artery lies behind it. The latter is 

 thus covered by the head of the pronator teres muscle which 

 extends upwards, and the condition resembles that of those 

 Mammals in which the brachial artery and median nerve pass 

 through an invariably developed foramen supracondyloideum. 1 



A comparison of the arteries of the hand with those of the 

 foot shows that there are in the hand two palmar arches, a 



to the recent series of very careful studies by Hochstetter, published in the Morpho- 

 logisches Jahrbuch. 



1 For further details on this point, cf. Huge, Morpholg. Jahrb., Bd. ix. p. 329. 



