REPTILES 229 



widely distended. In the snakes, moreover, the facial bones are very 

 movable. (Why ? See under snakes.) 



4. Body Temperature. The body heat (see Part L, p. 7) of reptiles 

 is considerably lower than that of birds or mammals. This is due to the 

 fact that 



(a) Respiration and (b) the circulation of the blood proceed much 

 more slowly than in the last-named classes. We know, however, 

 that in proportion as an animal breathes more rapidly and deeply, more 

 oxygen is conveyed to its blood, and that combustion (production of 

 heat) goes on more actively in proportion as the rapidity of the circulation 

 increases. To this must be added the important fact that the two ven- 

 tricles of the heart are not completely separated (in the crocodiles alone 

 is this separation perfect). Consequently, the arterial is mixed with the 

 venous blood, so that it is never pure (free from carbonic acid gas), 

 and carries less oxygen to the different parts of the body than is the case 

 in warm-blooded animals. 



(c) With a slowly-proceeding combustion, the quantity of fuel required 

 by the animal machine is inconsiderable (see Part I., p. 8). Eeptiles 

 accordingly need a much less amount of food than warm-blooded animals. 

 In relation with this, again, is the imperfect development of the organs 

 of locomotion in reptiles, and the general sluggishness of all their move- 

 ments as compared with the higher vertebrates. (If a reptile required 

 the same amount of warmth, and consequently of food, as a bird or 

 mammal, it would soon perish, in consequence of its inferior powers of 

 locomotion.) With the diminished food requirements of reptiles we 

 must further connect such facts as their strong need of repose, their 

 tolerance of prolonged fasting, their ability to live for a long time without 

 breathing, as well as the feeble mental faculties noticeable in all the 

 members of this class. Indeed, in comparison with mammals and birds, 

 reptiles may be said to be leading only " half a life." 



(d) From the two last-named classes of vertebrates, reptiles (as well 

 as all other animals) differ as regards their body temperature in one 

 other important point. It is well known that in birds and mammals the 

 vital activities are seriously endangered whenever the temperature of 

 their body falls much below or rises above the normal. In reptiles, 

 on the other hand, the vital activities are not restricted to a definite 

 temperature of the body. Their temperature, in fact, rises and falls with 

 that of the air which surrounds them, or of the water or soil in which 

 they live. They are, in short, animals of variable temperature (or, less 

 accurately, cold-blooded animals). 



However, as is the case in all other so-called cold-blooded animals, 

 with the rise of the temperature of the surrounding medium the vital 



