ARTERIES OF REPTILES. 519 



the leg, divides into posterior and anterior tibial, terminating in the 

 digital arteries and capillaries of the webs. 



In Ophidian, Lacertian, and Chelonian Reptiles the ' bulbus ' 

 of the embryo heart becomes divided into three distinct tubes, 

 which remain closely united together by their outer fibrous tissue, 

 and covered anteriorly by the reflected serous layer of the peri- 

 cardium. The exteut of this union, or length of the ' conus 

 vasculosus,' is greatest in the Serpents. In the Python it may 

 exceed two inches in length ; and, when the serous and fibrous 

 tunics are dissected away, the origin of the pulmonary artery is 

 seen to the left, next to it is the origin of the left aorta, and to 

 the right of this, about an inch above the ventricle, the trunk of 

 the right aorta appears, which, as it diverges from the left, sends 

 off the single carotid artery. This artery is the remnant of the 

 anterior of three primitive vascular arches. The right aortic 

 arch and the left aortic arch, which unite behind and beyond the 

 pericardium to form the abdominal aorta, are the proceeds of the 

 middle primitive arches : the pulmonary artery is the issue of the 

 changes of the posterior or first pair of vascular arches. 



In the Lacertilia the extent of modification is somewhat less. 

 Looking on the sternal surface of the heart, the pulmonary trunk 

 is the foremost, the left aorta is the next, the right aorta is the 

 hindmost. The left and right aortic arches converge, and unite or 

 intercommunicate, behind and usually below or beyond the heart, 

 to form the abdominal aorta. In Lacerta ocellata, fig. 334, P 

 marks the origin of the pulmonary artery, which ascends and 

 divides : the left branch, P, passing in front of the left descending 

 aorta, with which it is connected by a ductus arteriosus, D, before 

 proceeding to the left lung, p, fig. 332 ; the right pulmonary 

 artery, fig. 334, p', passes behind the ' arterial cone,' and in front 

 of the right descending aorta, a', with which it communicates, 

 or is connected, by a ductus arteriosus before proceeding to the 

 right king. These ' ductus arteriosi ' exist in the Python, are 

 shown in the Tortoise, fig. 335, d, d, and in the Crocodile, 

 fig. 340, d, d'. The third primitive arterial trunk, called ' right 

 aorta,' divides into the right arch (below a' in fig. 334), and 

 into the common trunk of the two cephalic or carotid arteries, 

 ib. a, a*, describing the upper arches : the common trunk of 

 the brachial arteries is usually given off from the right aortic 

 arch. In Psammosaurus griseus the common trunk of the carotid 

 does not bifurcate until it has ascended the neck as far as the 

 origin of the bronchial tubes : and not until after the rio-ht aorta 

 has arched over the right bronchus does it send off, at an acute 



