11. 3- 29 



unconcocted food into the stomach, namely the mouth, and in 

 some animals the so-called oesophagus, which is continuous with 

 the mouth and reaches to the stomach, so must there also be 

 other and more numerous channels by which the concocted food 

 or nutriment shall pass out of the stomach and intestines into 

 the body at large, and to which these cavities shall serve as a 

 kind of mangerJ For plants get their food from the earth by 

 means of their roots ; and this food is already elaborated when 

 taken in, which is the reason that plants produce no excrement,* 

 the earth and its heat serving them in the stead of a stomach. 

 But animals, with scarcely an exception, and notably all such 

 animals as are capable of locomotion, are provided with a 

 stomachal sac,^ which is as it were an internal substitute for the 

 earth. They must therefore have some instrument which shall 

 correspond to the roots of plants,^" with which they may absorb 

 their food from this sac, so that the proper end of the successive 

 stages of concoction may at last be attained. The mouth then, 

 its duty done, passes over the food to the stomach, and there 

 must necessarily be something to receive it in turn from this. 

 This something is furnished by the blood-vessels,^^ which run 

 throughout the whole extent of the mesentery from its lowest 

 part right up to the stomach. A description of these will be 

 found in the treatises on Anatomy and Natural History.^^ Now 

 as there is a receptacle for the entire matter taken as food, and 

 also a receptacle for its excremental residue, and again a third 

 receptacle, namely the vessels, which serve as such for the blood, 

 it is plain that this blood must be the final nutritive material 

 in such animals as have it ; while in bloodless animals the same 

 is the case with the fluid which represents the blood. '^ This 

 explains why the blood diminishes in quantity when no food is 

 taken, and increases when much is consumed,^* and also why it 

 becomes healthy and unhealthy according as the food is of the 

 one or the other character. These facts, then, and others of a 

 like kind, make it quite plain that the purpose of the blood in 

 sanguineous animals is to subserve the nutrition of the body. 

 They also explain why no more sensation is produced by touching 

 the blood, than by touching one of the excretions or the food, 

 whereas when the flesh is touched sensation is produced. For 

 the blood is not continuous nor united by growth with the flesh, 

 but simply lies loose in its receptacle, that is in the heart and 

 650b. 



