670 DEVELOPMENT OF THE CIRC-ULATORY APPARATUS. 



to the gills on each side of the neck ; but in the human subject and 

 the quadrupeds, the branchial tufts are never developed, and the 

 cervical arches, as well as the trunks with which they are con- 

 nected, become modified by the progress of development in the 



following manner: 



Fig. 256. 



Fig. 257. 



Early condition of ARTERIAL SYSTEM: 

 showing the heart (1), with its two ascend- 

 ing arterial trunks, giving off on each side 

 five cervical arches, which terminate in the 

 vertebral arteries (2, 2). The vertebral arte- 

 ries unite below the heart to form the 

 aorta (3). 



Adult condition of ARTERIAL SYS- 

 TEM. 1,1. Carotids. 2, 2. Vertebrals. 

 3, 3. Right and left subclavians. 4, 4. 

 Right and left superior intercostals. 5. 

 Left aortic arch, which remains perma- 

 nent. 6. Right aortic arch, which dis- 

 appears. 



The two ascending arterial trunks on the anterior part of the 

 neck, from which the cervical arches are given off] become con- 

 verted into the carotids. (Fig. 257, 1,1.) The fifth, or uppermost 

 cervical arch, remains at the base of the brain as the inosculation, 

 through the circle of Willis, between the internal carotids and the 

 basilar artery, which is produced by the union of the two verte- 

 brals. The next, or fourth cervical arch, may be recognized in an 

 inosculation which is said to be very constant between the superior 

 thyroid arteries, branches of the carotids, and the inferior thyroids, 

 which come from the subclavians at nearly the same point from 

 which the vertebrals are given off. The next, or third cervical arch, 

 remains on each side, as the subclavian artery (3, 3). This vessel, 



