CIRCULATION IN REPTILES. 249 



of the lesser circulation being here only a branch of 

 the greater. 



Nothing is more curious or beautiful than the 

 mode in which Nature conducts the gradual tran- 

 sition of the branchial circulation of the tadpole, 

 into the pulmonary circulation of the frog. In the 

 former, the respiratory organs are constructed 

 almost exactly on the model of those of tishes, and 

 respiration is performed in the same manner as in 

 that class of animals : the heart is consequently 

 essentially branchial ; sending very nearly the 

 whole of its blood to the gills, the veins returning 

 from which (describing the course marked by the 

 dotted lines m, n, in the diagram), unite, as in fishes, 

 to form the descending aorta.* As the lungs are 

 developed, small arterial branches, arising from the 

 ascending aorta, are distributed to those organs ; 

 and in proportion as these arteries enlarge, the 

 branchial arteries diminish ; until, on their be- 

 coming entirely obliterated, the course of the blood 

 is wholly diverted from them, and flows through 

 the enlarged lateral trunks (o, p,) of which the 

 junction constitutes the descending aorta. This 

 latter vessel now receives the whole of its blood 

 directly from the heart; which, from being origin- 

 ally a branchial, has become a systemic heart. 



* The only circumstance in which this circulation differs from 

 that of fishes, is that noticed by Rusconi ; namely, that a part of the 

 blood which has entered the branchial arteries finds its way, by col- 

 lateral branches, into the branchial veins, before these unite to form 

 the descending aorta, and thus escapes the action of the air, and 

 mixes itself with that portion which has received its influence in the 

 branchise. (Observations anatomiques sur la Sirene, mise en paral- 

 lele avec le Prolee et le Tetard de la Salamandre aciuatique. A Pavie, 

 1837, p. 7). 



