DEVELOPMENT OF THE VASCULAR SYSTEM. 795 



for a certain period. But at the time of birth the placenta is detached, 

 and the lungs brought into play, with comparative suddenness; and 

 although the pulmonary circulation and respiration are not established 

 in full activity until an interval of some days has elapsed, yet the pla- 

 centa is at once withdrawn from the circulatory system, and its office is 

 assumed by the lungs, the skin, and the alimentary canal. 



The comparatively sudden changes which take place at birth have, 

 however, been already provided for by the gradual development of the 

 necessary organs. This is accompanied by corresponding alterations in 

 both the arterial and venous systems. 



Development of the Arterial System. At an earl} 7 period of develop- 

 ment, the main arterial trunks, after passing off from the anterior ex- 

 tremity of the heart, curve backward in two sets of nearly parallel 

 branches, toward the vertebral column, after which they again become 

 longitudinal, and receive the name of the "vertebral arteries." The 

 curved branches which pass along the sides of the neck, from front to 

 rear, are called the cervical arches. They run in the substance of the 

 visceral folds existing in this situation (page 781), and are separated 

 from each other by the intervening cervical fissures. In the chick- 

 embryo, according to Foster and Balfour, three cervical arches, in the 

 three upper visceral folds, have been formed by the end of the second 

 day of incubation. During the third and fourth days, the first and 

 second cervical arches become obliterated, but a fourth and a fifth be- 

 come developed at the same time, in the substance of 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 bloodvessels sup- 

 plying the gills, generally four in number on each side, sometimes five. 

 In birds and mammalians, some of them disappear during the further 

 progress of development, or leave only certain arterial inosculations in 

 the adult, as vestiges of their existence during the embryonic condition. 

 Some of them, on the other hand, remain as permanent vascular trunks 

 or branches, forming important parts of the adult arterial system. 



The details relating to the growth and subsequent modification of 

 the cervical arches are not all described in the same manner by different 

 observers ; and there seems to be some variation, in this respect, in the 

 mammalian embryo, as compared with that of birds. The general fea- 

 tures, however, of the process of transformation 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 as above mentioned, or remain only in the form of small 

 and inconstant arterial inosculations. The third arch becomes the sub- 

 clavian artery, giving off, in an upward direction, the permanent verte- 

 bral artery, and continuing outward as the axillary artery, to supply 

 the upper limb. The fourth cervical arch undergoes very different 

 changes on the two opposite sides. On the left side it becomes enor- 



