RESPIRATORY CIRCULATION IN REPTILES. 275 



as usual, constitute the venae cavse (c), and ter- 

 minate in the auricle (d). 



From each of the trunks which arise from the 

 primary division of the aorta, there proceed the 

 small arteries (f), which are distributed to the 

 lungs (g, h), and convey to those organs a part 

 only of the mass of circulating blood. To 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 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 

 on the model of those of fishes, and respiration 

 is performed in the same manner as in that class 

 of animals : the heart is consequently 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 aorta, are 



