DEVELOPMENT OF THE VASCULAR SYSTEM. 691 



by the end of the second day of incubation. During the third and 

 fourth days, the first and second cervical arches become obliterated, 

 a fourth and a fifth appearing at the same time, in the corresponding 

 visceral folds. Thus there are, in all, five vascular cervical arches ; 

 but only three are to be found coexisting at any one time. 



In fishes, the cervical arches remain as permanent blood-vessels sup- 

 plying the gills, four or five in number on each side. In birds and 

 mammalians, some of the cervical arches disappear, or leave only cer- 

 tain arterial inosculations as vestiges of their embryonic existence. 

 Some, on the other hand, remain as permanent vascular trunks or 

 branches, forming important parts of the adult arterial system. 



The details of growth and modification in the cervical arches are not 

 all described in the same manner by different observers ; and there 

 seems to be some difference, in this respect, between mammalians and 

 birds. The general features of the process, however, are as follows : 



The two ascending trunks, on the anterior part of the neck, from 

 which the cervical arches are given off, become the carotid arteries. 

 The first and second, that is, the two upper cervical arches, on each 

 side, disappear completely, or remain only as small and inconstant arte- 

 rial inosculations. The third arch becomes the subclavian artery, giv- 

 ing off the permanent vertebral artery, and continuing its course as 

 the axillary artery, to the upper limb. The fourth cervical arch 

 undergoes different changes on the two sides. On the left side it 

 becomes enormously enlarged, giving off, as secondary branches, all the 

 arterial trunks going to the head and upper limbs, and is thus converted 

 into the arch of the aorta. On the right side it grows smaller, and 

 ultimately disappears ; so that at last there is only a single aortic arch, 

 situated on the left of the median line, and continuous below with the 

 thoracic aorta. 



The fifth or last cervical arch becomes on each side the pulmonary 

 artery ; its external portion on the right side disappearing at a very 

 early period, but on the left remaining for a certain time, as the ductus 

 arteriosus, between the pulmonary artery and the aorta. 



Notwithstanding that all the cervical arches are at first, as their 

 name implies, situated in the region of the neck, their remains or per- 

 manent representatives in the complete form of the arterial system, 

 come to be placed farther downward, and even in the cavity of the 

 chest. This is due to the varying rapidity of growth in different parts, 

 at successive periods of development. The thorax at first has no exist- 

 ence as a distinct portion of the trunk ; the heart being placed imme- 

 diately beneath the head, and afterward changing its position with the 

 increasing development of the lungs and chest. The neck, with the 

 oesophagus and trachea, also elongates in an upward direction, so that 

 the vascular organs of this region afterward occupy a situation farther 

 down. In fishes, where there are no lungs and no thoracic cavity, the 

 cervical arches are permanent, and the heart remains in the anterior 

 portion of the trunk, just behind the gills. 



