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 temperature 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
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-