28 ii. 3. 



is essentially hot in so far as that heat is connoted in its name ; 

 just as if boiling water were denoted by a single term, boiling 

 would be connoted in that term. But the substratum of blood, 

 that which it is [in substance] while it is blood [in form] is not 

 hot. Blood then in a certain sense is essentially hot, and in 

 another sense is not so. For heat is included in the definition 

 of blood, just as whiteness is included in the definition of a white 

 man, and so far therefore blood is essentially hot. But so far as 

 blood becomes hot from some external influence, it is not hot 

 essentially.^ 



As with hot and cold, so also is it with solid and fluid. We 

 can therefore understand how some substances are hot and fluid 

 so long as they remain in the living body, but become perceptibly 

 cold and coagulate so soon as they are separated from it ; while 

 others are hot and consistent while in the body, but when with- 

 drawn undergo a change to the opposite condition, and become 

 cold and fluid. Of the former blood is an example ; of the 

 latter bile ; for while blood solidifies when thus separated, yellow 

 bile under the same circumstances becomes more fluid.^ We 

 must attribute to such substances the possession of opposite 

 properties in a greater or less degree. 



In what sense, then, the blood is hot and in what sense fluid, 

 and how far it partakes of the opposite properties, has now been 

 fairly explained. Now since everything that grows must take 

 nourishment, and nutriment in all cases consists of fluid and 

 solid substances, and since it is by the force of heat that these 

 are concocted^ and changed, it follows that all living things, 

 animals and plants alike, must on this account if on no other 

 have a natural source of heat. This natural heat moreover must 

 belong to many parts,* seeing that the organs by which the 

 various elaborations of the food are effected are many in number. 

 For first of all there is the mouth and the parts inside the mouth, 

 on which the first share in the duty clearly devolves, in such 

 animals at least as live on food which requires disintegration. 

 The mouth, however, does not actually concoct the food, but 

 merely facilitates concoction ; ^ for the subdivision of the food 

 into small bits facilitates the action of heat upon it. After the 

 mouth come the upper and the lower abdominal cavities,^ and 

 here it is that concoction is effected by the aid of natural heat. 

 Again, just as there is a channel for the admission of the 

 650 a. 



