PARTS OF ANIMALS, II. iii. 



substances while in the Hving organism are hot and 

 fluid, but when separated from it congeal and are 

 observed to be cold, as blood does ; others, like 

 yellow bile, are hot and of a thick consistency while 

 in the organism, but when separated from it undergo 

 a change in the opposite direction and become cool 

 and fluid. Blood becomes more sohd, yellow bile 

 becomes fluid. And we must assume that " more 

 and less " ° participation in opposite characteristics 

 is a property of these substances. 



We have now pretty well explained in what way 

 blood is hot, in what way it is fluid, and in what 

 way it participates in opposite characteristics. 



Everything that grows must of necessity take food. 

 This food is always supplied by fluid and solid matter, 

 and the concoction ^ and transformation of these is 

 eifected by the agency of heat.'' Hence, apart from 

 other reasons, this would be a sufficient one for 

 holding that of necessity all animals and plants 

 must have in them a natural source of heat ; though 

 there are several parts which exert action upon the 

 food. In the case of those animals whose food needs 

 to be broken up, the first duty clearly belongs to 

 the mouth and the parts in the mouth. But this 

 operation does nothing whatever towards causing 

 concoction : it merely enables the concoction to 

 turn out successfully ; because when the food has 

 been broken up into small pieces the action of the 

 heat upon it is rendered easier. The natural heat 

 comes into play in the upper and in the lower gut, 



stance," as emphasizing its proper and specific natural 

 character, which makes it a particularly good agent for 

 effecting concoction. See Introduction, pp. 30-32. 



133 



