CLASS THIRD—REPTILES. 
Vertebrated animals with cold red blood, respuring by lungs; body naked er 
covered with scales. 
In reptiles, the heart is so disposed, that at each contraction, only a pore 
tion of the blood which it receives is conveyed to the .ungs; 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 Jand. 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 toa 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 after 
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 
ody itself continues to move for a still longer period, after the removal ot 
this essential organ. 
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