THE VASCULAR SYSTEM 333 



forms, where the first two become lost, these begin to degen- 

 erate before the more posterior ones appear. In the mam- 

 . malian embryo there are in the appearance of these arches 

 two important points to notice ; first, the successive supremacy 

 in size and function of each arch down to the fourth, and, 

 second, the extremely rudimentary condition of arch V, 

 amounting in some cases to a complete suppression. This is 

 shown in the rabbit embryo in the four states given in Fig. 92. 

 In a arch I is the principal, or, in fact, the only functional 

 one, and II and III are forming from approximated dorsal 

 and ventral buds. In b arch I has become disintegrated, 

 while the chief function is assumed by arch II: the ventral 

 bud of arch IV is also seen. In c both first and second arches 

 are lost and their remnants appear in part as continuations of 

 dorsal and ventral aortse and in part as stumps of vessels from 

 which important branches of the carotid system are to be de- 

 veloped. Arch V appears at about its maximum here and in 

 the next figure, and the ventral bud of arch VI has become 

 well developed. In d arch IV, to be later the permanent 

 aortic arch, is assuming good proportions, and arch VI, the 

 future pulmonary arch, is completed. From the dorsal side 

 of the dorsal aorta appear the beginnings of certain transitory 

 arteries that correspond to the head somites. 



The complete ontogenetic history of the carotid system in 

 mammals shows a curious shifting of branches from one source 

 to another, and the development and decay of transitory ele- 

 ments. In this the remnants of arches I and II, long sup- 

 posed to be lost, play a prominent part, and unite with the main 

 carotid arch (III) in the formation of the system. This 

 history, together with that of the other arterial arches, is seen 

 in the accompanying set of diagrams (Figs. 93 and 94), based, 

 with the exception of the last one, upon the embryology of the 

 rat. In a is seen the first arch [Cf. Fig. 89, a], which starts 

 from the conus arteriosus (ca), proceeds forward, and re- 

 turns as the dorsal aorta (ad), giving forth a carotid, the 

 arterla carotis cerebralis (ccb), just before returning. The 

 ventral bud of arch II has also appeared. In b arch II has 



