THE REPTILES 197 
remain functional throughout life. Corresponding to the 
shape of the body, these are usually much elongated and 
ordinarily paired (Fig. 118, /./ng.). The snakes are peculiar 
in having the left lung rudimentary or even lacking com- 
pletely, while the right one becomes greatly elongated and 
extends far back into the body. In nearly all the reptiles 
the amount of oxygen brought into the lungs is relatively 
large and the activity of the animal is proportionately 
great. The circulation of reptiles shows an advance be- 
yond that of the Amphibia. As in the latter, there are 
two distinct auricles; but the chief difference arises from 
the fact that the ventricle is more or less divided by a par- 
tition which to a considerable degree prevents the blood 
returning from the lungs from mixing with the impure 
blood as it returns from its journey over the body. In the 
crocodiles and alligators the partition is complete, and the 
circulation thus approaches close to that of the higher 
animals. 
186. Hibernation.—Attention has already been called to 
the fact that reptiles are very susceptible to cold, rapidly 
growing less active as the temperature lowers. When win- 
ter comes on they seek protected spots, and either alone 
or grouped together hibernate. The various activities of 
the body during this period are at very low ebb. The blood 
barely circulates, breathing is imperceptible, and _ stiff 
and insensible to the world about them they remain until 
the warmth again stirs them to their former activity. 
Some of our common turtles must also pass a somewhat 
similar sleep while embedded far down in the mud during 
the disappearance of the ponds in summer. At such times 
no food is taken, but owing to their loss in weight it is 
probable that a slow consumption of the body supplies the 
small amount of necessary energy. 
187. Nervous system and sense-organs.— At first sight one 
is struck with the small size of the brain of fishes, Am- 
phibia, and reptiles. Their intelligence likewise is at low 
