520 ANATOMY OF VERTEBRATES. 



angle, the common trunk of the right and left brachials. The 

 left aortic arch, in Psammosaurus, sends off a gastric and mesen- 

 teric artery before it joins the right aortic arch ; and this is the 

 case also in the Tortoise, fig. 335, I, and Crocodile, fig. 339, h, 

 the left aorta being then so reduced in size as to resemble a 

 ' ductus arteriosus.' 



In the Tortoise the right aorta, soon after its origin, sends off 

 a common trunk, which quickly subdivides into the carotids, 

 fig. 335, a*, a*, and brachials, ib. , a ; and the same is the case 

 with Enujs, in which the four arteries are seen cut short near their 

 origins, between A' and A, figs. 337 and 338. 



In the Crocodile the two ( arteriaa innominate,' figs. 339, 340, b,b, 

 are longer before they divide into the brachial, , and carotid, a' : 

 both innominata3 arise by a short common trunk from the right 

 aorta, which divides, soon after its origin, into that trunk and the 

 right aortic arch, ib. A'. This arch winds over the right pul- 

 monary artery, fig. 339, p', with which it is connected by a 

 ' ductus arteriosus ' (the arch is reflected upward and the ' ductus ' 

 D', severed from the pulmonary artery, p', in fig. 340). The 

 origin of the right or brachi-cephalic aorta is hidden by that 

 of the left aorta, fig. 339, A, which is anterior, or on the sternal 

 side of it. The left aorta, as it winds over the left pulmonary 

 artery, is attached to it by a ' ductus arteriosus,' the remnant of 

 the channel by which the first vascular arch originally com- 

 municated with the second, to aid in forming the aorta, before its 

 current of blood was diverted to the uses of the well-developed 

 lung, after exclusion. The continuation of the main part of the 

 left aorta into the great visceral artery, fig. 339, A, reduces its 

 original union with the right aortic arch, A', A" ', to a small anas- 

 tomotic channel. 



The following particulars are notable with regard to the dis- 

 tribution of arterial blood by the right aorta in Reptiles. The 

 anterior vascular arch, in Ophidia, is converted into a pair of 

 cephalic or carotid arteries in the young Snake, and this structure 

 is retained in the common Snake ( Coluber natrix) ; in Python 

 tic/ris I found the right carotid much reduced in size ; in the 

 Viper ( V. verus\ and some other Serpents, the cervical part of 

 the right carotid is obliterated : but the cephalic portion remains, 

 and receives blood by the anastomosis of one of its branches 

 with the left carotid. 1 In the Viper, and some other venomous 

 Serpents, the internal maxillary branch of the carotid forms a 

 rete mirabile behind the poison-gland. 2 The right aortic arch, 



1 CCLXXXl. 2 CCLXXXII . p. 260. 



