198 THE VITAL FUNCTIONS. 



these pulmonary arteries there correspond a set of veins, 

 uniting in the trunks (i,) which bring back the aerated blood 

 to the auricle of the heart (D,) where it is mixed with the 

 blood which has returned by the venae cavae (c,) from the 

 general circulation. Thus the blood is only partially aerated, 

 in consequence of the lesser circulation being here only a 

 branch of the greater. 



Nothing is more curious or beautiful than the mode in 

 which, during the transformations of this animal, Nature 

 conducts the gradual transition of the branchial circulation 

 of the tadpole, into the pulmonary circulation of the frog. 

 In the former, the respiratory organs are constructed on the 

 model of those of fishes, and respiration is performed in the 

 same manner as in that class of animals: the heart is conse- 

 quently essentially branchial, sending 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 develope, small arterial branches, arising from the aor- 

 ta, are distributed to those organs, and in proportion as these 

 arteries enlarge, the branchial arteries diminish; until, on 

 their becoming entirely obliterated, the course of the blood 

 is wholly diverted from them, and flows through the en- 

 larged 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 be- 

 ing originally a branchial, has become a systemic heart. 



The heart of the Chelonian reptiles, such as the ordinary 

 species of Tortoises and Turtles, has two distinct auricles; 

 the one, receiving the blood from the pulmonary veins; the 

 other, from those of the body generally; so that the mixture 

 of aerated and vitiated blood takes place, not in the auricle, 

 but in the ventricle itself. When all the cavities are dis- 

 tended with blood, the two auricles being nearly of the same 

 size as the ventricle, the whole has the appearance of a union 

 of three hearts. The circulatory system of the Ophidia is 

 constructed on a plan very similar to that of the Chelonia. 



