PARTS OF ANIMALS, II. ix.-x. 



class too. In some creatures teeth are present to 

 discharge one function only — viz. mastication ; in 

 others they are a means of force as well (e.g. 

 sawhke teeth and tusks). All these parts are 

 of necessity earthy and solid in character ; that 

 is the proper sort of substance for a weapon. 

 So there is a tendency for all parts of this sort to 

 appear in the four-footed \^ivipara more extensively 

 than in man, because the former all have more earthy 

 matter in their constitution. We shall, hoΛvever, con- 

 sider these substances, and the other kindred ones 

 such as slvin, bladder, membrane, hair, feather, and 

 the counterparts of them, and all such parts, when 

 we come to deal with the non-uniform parts. Then 

 also we shall consider the Causes of them and for 

 what purpose each of them is present in animal bodies; 

 since it is true to say, of both sets of things, that our 

 knowledge of them must be derived from a study of 

 the functions which they discharge. The reason why 

 we have just been taking them Avith the uniform 

 substances and out of their proper order is that in 

 them the name of the complete structure is the same 

 as that of a portion of it, and also because the sources 

 and principles of them all are bone and flesh. We 

 also left out all mention of semen and milk Avhen we 

 were considering the fluid uniform substances. As 

 semen is the source of the things that are generated 

 and milk is the food that feeds them, the proper place 

 to discuss these is in the treatise dealing with 

 Generation. 



X. We may now make Avhat is practically a fresh The non- 

 beginning. We Avill begin first of all with the things ^^^j^^"» 

 that come first in importance. 



171 



