MORPHOLOGY OF THE ORCAXS OF VERTEBRATES. 



nected with the radix, so that both arches are aortic in character, 

 while the blood going through the third arch in part goes to 

 the carotids. In the other reptiles the connection between the 

 third and fourth arches disappears (Fig. 200 C), and here the 

 third arch is purely carotid in character. The right arch of 

 the fourth pair forms the main trunk of the dorsal aorta, while 

 the left, which as we have seen is connected with the right side 

 of the heart, is largely distributed to the 

 digestive organs, only connecting with the 

 dorsal aorta by a small trunk (Fig. 198). 

 As a result of this distribution of vessels 

 the carotids and the dorsal aorta receive 

 arterial blood, while the stomach receives 

 only venous or mixed blood. The re- 

 mainder of the venous blood goes through 

 the fifth arch to the lungs. 



In the birds and mammals where there 

 are but two arterial trunks, all of the venous 

 blood goes to the lungs, all parts of the 

 sj'stemic arteries receiving arterial blood. 

 The chief distinction between these groups 

 hes in the fact that in the birds the right 

 half, in mammals the left, of the fourth 

 arch persists (Fig. 200 D and E). 



Arteries. — As development proceeds 

 other arteries than those mentioned on 

 p. 183 arise from the dorsal aorta and its 

 radices. The chief of these are the follow- 

 ing. In the cervical and posterior cranial 

 regions are as many segmental arteries as 

 there are segments. These are united by 

 anastomoses on either side, after which the 

 roots of the segmental arteries themselves disappear, with the 

 exception of the last, which remains as the stem, while the 

 anastomosing vessel on either side persists as the vertebral ar- 

 tery, growing forward into the head, where it anastomoses with 

 the carotids. From the stem of the vertebral artery the sub- 

 clavian artery arises as a bud, and with the outgrowth of the 



Fig. 201. Diagram 

 of the circulation in a 

 mammal, the arterial 

 parts white, the venous 

 shaded ; the arrows 

 show the direction of 

 the flow of blood, a, 

 auricles ; /, lung ; Iv, 

 liver; p, portal vein; 

 T, veins. 



