ORGANIZATION OF REPTILES. 21 



nicating with the general cavity. In the Sauria the heart is ovoid in shape, similar 

 in its general structure to that of the Chelonia, but more complicated in the arrange- 

 ment of its parts. In the Alligator there are two ventricles, entirely separate,* 

 so that the venous and arterial blood are only mixed in the sac of the great vessels. 

 In Serpents the heart is of more simple formation; the two auricles open into a 

 single ventricle, subdivided into cavities communicating with each other. In the 

 Batrachia we meet with a remarkable and entirely different arrangement of the 

 heart, unlike that of any other of the class; the organ is single, consisting of a 

 large auricle with thin, and a single ventricle with thicker parietes; a single opening 

 in the auricle communicates with the ventricle, and one opening in the ventricle 

 is common to the arteries. This simple structure of the heart has been denied by 

 Davyt and by Weber:;}: they describe the auricle as subdivided into two cavities 

 by a transparent membrane, which is certainly not the case in any of the American 

 Frogs that I have examined. 



Physiology of Circulation. — From this structure of the circulatory organs of 

 Reptiles, it results that only a moiety of the blood of the system can pass to the 

 lungs to be exposed to atmospheric air, whUe in the Mammalia the whole mass is 

 offered to its influence; and further, that there must be an admixture in the single 

 ventricle of the blood of the two auricles, or of the blood returning from the lungs 

 on one side, and of that portion coming from the body on the other, which has not 

 been carried to the cells of the lungs; and thus a portion only of the whole mass 

 can be decarbonised and fitted for nutrition. Hence we do not observe as great 

 difference in the external appearance of venous and arterial blood in Reptiles, as 

 in the Birds and Mammalia. 



5. Organs of Respiration. — No organic body can live without air; neither 

 animal nor plant. Some beings derive it directly from the atmosphere, others 

 from the water; and the respiratory organs vary according to the mode of life of 



* Hentz, Amer. Phil. Trans., n. s., vol. vii. p. 222. 



t Zool. Journal, vol. ii. :]: Beitrag. von dem Herzen. 8vo. 1S33. 



