ORDER CHELONIA. 9.5 



relation to the wants of the internal organs ; the latter must, 

 therefore, be investigated, if we want to ascertain the cause 

 which determines the conformation of the former. 



Moreover, there are in each class of animals, and of 

 plants, certain parts which may be said to give the impulse 

 to the whole organized machine, and certain organs which 

 assume an ascendancy over the other organs, in consequence 

 of the extent and the energy of their functions. Thus, the 

 pulmonary apparatus is predominant in birds ; in man, the 

 brain and the nervous system ; and in the camivora, the 

 muscular system, because the said organs in these different 

 animals are more developed and more active than all the 

 rest. In reptiles and in fishes, it is the muscular contrac- 

 tility which claims pre-eminence, above all other properties, 

 for its influence and duration. 



But if it is essential to recognize the predominating organs 

 in any class of animals, it is equally important to study those 

 which are feeble and comparatively inactive, because they 

 produce effects inverse of those of the former in the living 

 economy. The knowledge, in fact, of the one, necessarily 

 determines the others, and weakness thus becomes as influen- 

 tial as strength on the animated machine. In truth, it is to 

 the imperfect state of the respiratory apparatus that the 

 entire constitution of reptiles must be referred. 



These animals all respire the air because they have lungs. 

 But this organ is vesicular, and the blood vessels which 

 arrive at it, are only branches of the vena cava and the aorta, 

 so that tliose vessels form no considerable system, and tran- 

 sport but a small quantity of blood to the lungs, instead of a 

 mass of this fluid almost equal to that of the rest of the 

 body, as in warm-blooded animals. For this same reason 

 the heart of reptiles possesses but a single ventricle, which 

 suffices to make their blood circulate, independently of respi- 

 ration. The latter may remain suspended for some time 



