12 REPTILES AND BIRDS. 



The respiration of Reptiles and some of the Batrachians, like 

 that of Birds and Mammals, is aerial and pulmonary, but it is much 

 less active. Batrachians have, in addition, a very considerable 

 cutaneous respiration. Some of them, such as Toads, absorb more 

 oxygen through the skin than by the lungs. Their circulation is 

 imperfect, the structure of the heart only representing one ventricle ; 

 the blood, returning after a partial regeneration in the lungs, mingles 

 with that which is not yet revivified : this mixed fluid is launched 



Fig. 2. — Skeleton of a Frog. 



out into the economic system of the animal. Thus Reptiles and 

 Batrachians are said to be cold-blooded animals, more especially the 

 former, in which the respiratory organs, which are a constant source 

 of interior heat, are only exercised very feebly. Owing to this low 

 temperature of their bodies, Reptiles affect warm climates, where 

 the sun • exercises its power with an intensity unknown in tem- 

 perate regions ; hence it is that they abound in the warm latitudes 

 of Asia, Africa, and America, whilst comparatively few are found 

 in Europe. This is also the cause of their becoming torpid in our 

 latitudes during the winter, not having sufficient heat in themselves 

 to produce reaction against the external cold, re-awakening only 



