DEVELOPMENT OF THE CIRCULATORY SYSTEM. 



433 



FIG. 51. 



right is obliterated ; the third, on either side, is converted into 

 the right and left pulmonary arteries. In the early stages of 

 the development of the vascular system of mammals, the 

 conditions have been compared to the permanent arrange- 

 ment of the circulatory system in fishes. The heart of fishes 

 remains single ; and the heart of mam- 

 mals is at first single, but afterward be- 

 comes divided, by the development of 

 the intra-ventricular septum. The bran- 

 chial arches in fishes are permanent, re- 

 ceive all the blood from the aortic bulb, 

 and the blood from these arches then 

 passes into the dorsal aorta. This is 

 veiy nearly the condition of the vascular 

 system when the branchial arches first 

 appear in the embryon of mammals. 



The changes of the branchial arches 

 which we have described are illustrated 

 in the diagrammatic Fig. 51. In this 

 figure, the three branchial arches that 

 remain and participate in the develop- 

 ment of the upper portion of the vascu- 

 lar system are 1, 2, 3. The two ante- 

 rior (3) become the carotids (c) and the 

 subclavians (s). The second (2) is ob- 

 literated on the right side, and becomes 

 the arch of the aorta on the left side. 

 The third (1), counting from above 

 downward, is converted into the pulmo- 

 nary arteries of the two sides. Upon the 

 left side, there is a large anastomosing 

 vessel (ca\ between the pulmonary artery of that side and 

 the arch of the aorta, which is the ductus arteriosus. The 

 anastomosing vessel (od) between the right pulmonary artery 

 and the aorta, is obliterated. 



1 Von Baer described five arches, while we have adopted but four. Only 

 three of these arches, however, remain to be developed into permanent vessels. 



Transformation of the system 

 of aortic arches into per- 

 manent arterial trunks, in 

 the mammalia, after Von 



' Baer. B, aortic bulb; 1, 

 2, 3, 4, 5, on each side, the 

 five pairs of aortic arch- 

 es ; i 5, the earliest in their 

 appearance; 1, tha most 

 recent ; c, c, the two carot- 

 ids, still united, which are 

 separated at a later pe- 

 riod ; , s, the two subcla- 

 vians, the rifrht arising 

 from the arteria innomi- 

 nata ; , a, the aorta ; />, p, 

 the pulmonary arteries; 

 ccr, the left arterial canal, 

 which is finally obliterat- 

 ed; ccf, the ductus arte- 

 riosus. (LONGET, Traite 

 de physiofoffie, Paris, 

 1869, tome iii., p. 984.) 



