REPTILIA. 571 



main vascular trunks are supported. From the heart, situated upon 

 the opposite side of the oesophagus, is given off a large vessel 

 representing the bulbus arteriosus of fishes, which terminates by 

 dividing into four branchial arteries ; but, as in the adult Meno- 

 poma there are no branchiae, these vessels (o, o, o) wind round each 

 side of the neck, and again unite into two trunks (r, r) which by 

 their union form the aorta (t, t). It will easily be perceived that 

 this arrangement is precisely that met with in fishes ; only that, as 

 there are here no gills intervening between the terminations of the 

 branchial arteries and the commencements of the branchial veins, 

 these vessels are immediately continuous with each other. Moreover, 

 from the lowest branchial arch (o) a pulmonary artery is given off, 

 which ramifies over the surface of the as yet rudimentary lung (e), 

 and thus gives rise to a distinct pulmonary circulation. 



Having carefully considered the disposition of the vessels in the 

 Menopoma above described, the reader will be able to appreciate 

 the arrangement of the vascular system in those Amphibia which, 

 being provided both with gills and lungs through the whole of their 

 lives, literally combine the blood-vessels of a fish with those of an 

 air-breathing reptile. 



In the PERENNIBRANCHIATA, as, for example, in the Proteus, 

 instead of the bulbus arteriosus being immediately continuous with 

 the aorta, as it is in the Menopoma, through the interposition of 

 the vessels o, o, o, (Jig. 257,) the blood derived from the heart 

 is obliged to pass more or less completely through gills appended 

 to the sides of the neck before it arrives in the vessels (r, r), 

 which may be said to represent the branchial veins of fishes. 



The branchiae are either vascular tufts or pectiniform organs, 

 (fig. 258, b, b,) essentially analogous in structure to those of a fish. 

 The blood, however, which is propelled from the heart is not here 

 entirely venous, but consists of a mixed fluid, partially derived 

 from the systemic and p- lg , 253. 



partially from the pul- 

 monary auricle, the two 

 having of course been 

 mingled together in the 

 common ventricle of the 

 tripartite heart. The con- 

 traction of the heart forces 



the blood into the bulbus arteriosus, from which it is in great part 

 driven into the branchiae : arrived there, it passes along the great 



