372 



EBIBRYOLOGY. 



the visceral arches retaining their original simple condition. But 

 they are in part of only short duration; they soon suffer, by the 

 complete degeneration of extensive portions, a profound metamor- 

 phosis, which is effected in a somewhat different manner in Reptiles, 

 Birds, and Mammals. An exposition of the changes in the case of 

 Man only will be given here. 



In human embryos only a few millimetres long, the truncus 

 arteriosus, which emerges from the still single cardiac tube, is divided 

 in the vicinity of the first visceral arch into a left and a right 

 branch, which surround the pharynx, and are continuous above with 

 the two primitive aortse. It is the first pair of aortic arches. In 



Fig. 317. Development of the large arterial trunks, represjntedfrom embryos of a Lizard (A), 



the Chick (B), and the Pig (C), after RATHKE. 

 The first two pairs of arterial arches have in a'l cases disappeared. In A an 1 B the thiiv, 



fourth, and fifth pairs are still fully preserve.! ; in C o-.i'y the two latter are s'ill coniple e. 

 p, Pulmonary ar;ery arising from the fifth arch, but s.ill joiue.l to the dorsal aor;a by n.fu: B <>f 



a ductus Botalli ; c, external, c', internal carotid ; ad, dorsal aorta ; a, atrium ; v, ventricle J 



n, nasal pit ; m, fundament of the anterior limb. 



only slightly older embryos their number is rapidly increased by 

 the formation of new connections between the ventral truncus 

 arteriosus and the dorsal primitive aortfe. Soon a second, a third, 

 a fourth, and, finally, a fifth pair make their appearance in the 

 same sequence in which the visceral arches are established in the 

 case of Man as well as the remaining Vertebrates. 



The five pairs of vascular arches give off lateral branches to 

 the neighboring organs at a very early period ; of these several 

 acquire a great importance and become carotb- externa and interna, 

 vertebralis and subclavia as well as pulmonalis. The carotis externa 

 (fig. 316 ce and fig. 317 c) arises from the beginning of the first 

 vascular arch, and is distributed to the region of the upper and 



