244 ZOOLOGY. 
VERTEBRATA. 
Cuass IJ. Repritta. 
The Reptilia are cold-blooded vertebrates like fishes, but are distinguished 
from these by the pulmonary respiration, the heart with three chambers, 
the presence of organs of motion other than fins, and by various other 
points of organization. 
The circulation of the blood is incomplete ; less complete even than in. 
fishes. The heart consists of two auricles and one ventricle. The 
venous blood, collected from the various parts of the body, accumulates in 
the vena cava, and thence passes into the right auricle. From the right 
auricle it passes into the single ventricle, and by it is impelled through the 
aorta into all parts of the body. A small branch leads to the lungs, and 
the blood when purified is returned to the left auricle, which drives it into 
the ventricle. This ventricle thus receives venous blood from one auricle, 
arterial from the other, and it is a mixture of this kind which is distributed 
through the body. The naked skin of the Batrachian reptiles serves a 
good purpose in the decarbonization of the blood, owing to the extensive - 
distribution of bloodvessels immediately on the under side of the skin. 
The blood of Reptilia is characterized by the possession of the largest 
globules to be found in the entire vertebrate sub-kingdom. These, in the 
tailed batrachians, as Siren, &c., are distinctly visible to the naked eye. 
As in fishes and birds these globules are elliptical in outline ; in mammaliia, 
with the single exception of the Camelide, they are circular. 
The lungs lie free in the abdominal cavity ; these, with the heart, not 
being separated from the other viscera by a diaphragm. ‘The cells of the 
lungs are of greater or less subdivision ; in many of the North American 
Salamandre they are mere sacs. Reptiles are better able to sustain the de: 
privation of oxygen than other vertebrates: this, however, depends greatly 
upon the temperature and season. Thus a frog will bear the deprivation 
of atmospheric air in summer for a space of time not much exceeding two 
hours, while in winter it can sustain its absence for several days. 
A point of great physiological interest in the structure of reptiles consists 
in the fact that some forms present, at different times of life, both fish-like 
and reptilian features of respiration. Thus the salamanders and frogs, when 
young, respire for a certain length of time, for years in some, by means of 
external gills, the lungs being entirely rudimentary. In course of time the 
lungs acquire a greater development, and the gilis disappear. This fish-like 
condition of things, transitory in some, is permanent in others, as in Meno- 
branchus, Siren, and Proteus, which throughout life possess external gills. 
So true is it that the skin in the naked reptilia is accessory to the 
function of respiration, that the experiment has been tried with perfect 
success as to how far respiration might be carried on entirely by means 
of the skin. Thus the lungs of the frog have been tied in such a manner 
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