THIRD GREAT DIVISION 



THE ANIMAL KINGDOM. 



CLASS III.-VERTEBRATA. 



REPTILES. 



Reptiles have the heart so contrived as that, at each of its contractions, 

 it transmits to the lungs a portion only of the blood which returns to it 

 from the various parts of the body, and the remainder of this fluid goes 

 back to circulate again, without having passed into the lungs, and conse- 

 quently without having been subjected to respiration. 



Hence it is, that the action of the oxygen on their blood is less than in 

 the Mammalia, and that if the amount of respiration in the latter, in 

 which all the blood is obliged to pass into the lungs before its return to 

 the body, be represented by unity, then the amount of the respiration of 

 the Reptiles may be expressed by a fraction of that unity, as small as the 

 proportion of the blood sent into the lungs at each contraction of the 

 heart. 



As respiration is the source of the heat of the blood, and of the sus- 

 ceptibility of fibrous structure to nervous irritability, the reptiles having 

 cold blood, and having a muscular power on the whole much inferior to 

 that of the Quadrupeds, and a fortiori, than the Birds, they are incapa- 

 ble almost of other motions than those of crawling and swimming: and 

 although several of them jump and run very fast at certain moments, yet 

 their habits are slothful, their digestion excessively tedious, their sensa- 

 tions blunt, and in cold or temperate countries they pass the winter in a 

 state of lethargy. Their brain, which is proportionally very small, is not 

 so necessary to the exercise of their animal and vital faculties, as to the 

 members of the two first classes ; their sensations seem to be less referred 

 to a common centre ; they continue to live and to exhibit voluntary motions 

 long after losing their brain, and even after their head has been cut off. 



VOL. n. B 



