PARTS OF ANIMALS, II. ix. 



has to be somewhat supple, and accordingly the 

 supporting framcAvork of their bodies must be some- 

 what pliable, not brittle. In addition. Nature cannot 

 allot the same plentiful supply of any one substance 

 to many different parts of the body ; <» and in the case 

 of the Selachia she has used up all the available earthy 

 substance in constructing their skin. In the Vivipara 

 too there are many instances of cartilaginous bones : 

 they are found where it is an advantage that the 

 solid framework should be pliable and glutinous for 

 the benefit of the flesh that surrounds them. This 

 applies to the ears and the nostrils. Such projecting 

 parts quickly get broken if they are brittle. Car- 

 tilage and bone are the same in kind and differ 

 only by " the more and less " ^ ; so neither of them 

 continues to grow when it has been cut out of the 

 hving organism. 



The cartilages of land-animals contain no marrow — 

 that is, no marrow existing as a separate thing. 

 What in ordinary bones is separable is here mixed 

 in with the body of the cartilage and gives it its 

 pliable and glutinous character. In the Selachia, 

 however, although the backbone is cartilaginous it 

 contains marrow, because it stands to these creatures 

 in place of a bone. 



The following substances or " parts " resemble 

 bones very closely as regards their feel : the various 

 sorts of nail ; hoof and talon ; horn, and beak. 

 All these substances are present for the sake of self- 

 defence. This is shown by the fact that the complete 

 structures which are made out of them and bear the 

 same names — e.g. the complete hoof, or horn — have 

 been contrived in each case by Nature for the creature's 

 self-preservation. We must reckon the teeth in this 



169 



