572 



EMBRYOLOGY. 



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. — SeTelopment of the large arterial trunks, represented from embryos of a Lizard (^), 



the Chick (B), and the Pig ((7), af bar Eathke. 

 The first two pairs of arterial arches have in all cases disappeared. In A and B the third, 



fourth, and fifth pairs are still fully preserved ; in C only the two latter are still complete. 

 p, Pulmonary arfceiy arising from the fifth arch, but still joined to the dorsal aorta hy means of 



a ductus Botalli ; c, external, c', internal carotid ; ad, dorsal aorta ; a, atrium ; Vj ventricle : 



w, 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 aortse. 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 oif lateral branches to 

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

 acquire a great importance and become carotis 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 



