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THE POPULAR SCIENCE MONTHLY. 



continuously at 60°, some of our common snakes become sluggish and 

 inactive. In both mammals and reptiles the source of internal heat 

 is the same, the difference being in degree only. The low tempera- 

 ture of serpents (as of other reptiles) arises from the structure of 

 their vital organs, by means of which their blood is imperfectly 

 oxygenized. As the " worn-out " or venous blood enters the heart, 

 it is mixed with the vitalized blood from the lung (there being, 

 in most species, only one lung and a rudiment of another), and it is 

 this mixed blood which is thrown into the general circulation, as shown 

 in Fig. 5. The blood of a serpent has been said for this reason to be 



Fig. 6. 



Anaconda. 



only half alive, and their functions are accordingly sluggish and dull. 

 Their power of existence for long periods without food, and with little 

 waste of tissue, is chiefly incident to their low vitality. 



Hibernation is with them a period of profound torpor. In our 

 temperate climates they gather in large numbers, in some hole, or bur- 



