332 HISTORY OF THE HUMAN BODY 



sented by sub-clavi<z secundarice, branches of the carotids, but 

 in mammals they are the primary vessels, homologous with 

 those of typical reptiles. There is, indeed, a new morpho- 

 logical distinction between those of the two sides, for while 

 the left one, that of the side which furnishes the aortic arch, 

 arises from this arch, that of the right is the equivalent, not 

 only of that of the left side, but of the fourth arch as well. 

 It is this relationship which causes the intimate association 

 upon the right side between the subclavian and carotid, and 

 the short common trunk, arteria anonyma [innominata], is 

 thus the region once common to arches III and IV. Thus, 

 while these vessels on the right side are superficially similar 

 in both birds and mammals, they are morphologically totally 

 different. In the former the " subclavian " is a secondarily 

 formed branch of the carotid, with the true subclavian prob- 

 ably suppressed ; in the latter the " subclavian " is the fourth 

 arch plus the true subclavian that once branched from the 

 point where this arch joined an aorta, as on the other side. 



In this relationship is seen also the explanation of the curious 

 asymmetry in the origin of the human carotids and subclavians, 

 a condition which is undoubtedly a primitive one. This be- 

 comes more complicated in many other more specialized mam- 

 mals by various secondary approximations and fusions. Thus, 

 in the Carnivora the left carotid fuses near its base with the 

 other and produces the phenomenon of three of the four ar- 

 teries in question arising from a common stump, while the left 

 subclavian is alone distinct : in ruminants this latter also shifts 

 its position forward and fuses with the others, so that a single 

 median trunk arises from the crest of the arch. This gives 

 off first the two subclavians, and then, after continuing for- 

 ward a little, divides into the two carotids. 



Ontogenetically the most anterior of the six arterial arches 

 is the first to appear, and this, with the ventral and dorsal 

 aortae, forms a lateral loop directed forwards. The other 

 arches are successively added through the formation along the 

 course of the aortae of buds or sprouts that meet and join. In 

 fishes all but the first of the arches are retained, but in higher 



