CLASS THIRD — REPTILES. 



Vertebrated animals with cold red blood, respiring by lungs; body naked or 



covered with scales. 



In reptiles, the heart is so disposed, that at each contraction, only a por- 

 tion of the blood which it receives is conveyed to the lungs ; and from this it 

 results that the action of oxygen on the blood is much less than in the mam- 

 miferous animals or birds, where all the blood is exposed to the action of the 

 air. As respiration gives the blood its heat, and muscular fibre its suscepti- 

 bility for nervous irritation, the temperature of reptiles is comparatively 

 lower, and their muscular strength weaker than that of quadrupeds, and 

 much less than that of birds. Their motions are chiefly confined to that of 

 crawling and swimming ; and though many at times leap and run very 

 quickly, yet their general habits are sluggish, their sensations obtuse, their 

 digestion slow, and in cold or temperate countries they pass almost the 

 whole winter in a state of torpidity. The heart is composed, in frogs, of an 

 auricle and a ventricle ; in serpents, of two auricles, and a ventricle of two 

 compartments ; and in the tortoises and lizards, of two auricles and a ventri- 

 cle of communicating cavities. The general resemblance in point of form, 

 which characterizes the two preceding classes, is not applicable to the pre- 

 sent class ; for while some, as the serpents, have no members at all, others 

 have two short legs ; and the lizards, tortoises, and frogs, have four, adapted, 

 in the two last, to progression in the water and on land. Neither is there a 

 common external covering for the class, as fur for the quadrupeds, or feathers 

 for the birds. Their low temperature, not differing much from the medium 

 in which they live, renders such a covering to retain the heat unnecessary. 

 The skin is naked in frogs, scaly in lizards and serpents, and covered with a 

 shelly plate in the tortoises. The brain in reptiles is small, their nerves are 

 very solid, and the relation of their sensations to a common centre seems less 

 necessary to their animal and vital functions than the higher classes. They 

 continue to live and possess voluntary motion for a considerable time aftei 

 the brain is removed, and even when their head is cut off. The connection 

 of the nervous system with the muscular fibre is also less necessary to its 

 contraction ; and their muscles preserve their irritability longer after being 

 separated from the body, than in the previous classes. Even in some species, 

 the heart beats many hours after it has been taken from the body, and the 

 body itself continues to move for a still longer period, after the removal ot 

 this essential organ. 



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