PARTS OF ANIMALS, II. viii.-ix. 



bone is and more bony and earthy than flesh. The 

 purpose of this is to ensure that the body shall not 

 easily break up. 



IX. The system of the bones is similar to that of Bones, 

 the blood-vessels : each is a connected system begin- 

 ning from one point. There is no such thing as a 

 bone by itself in isolation ; every bone is either 

 actually part of the connected scheme, or else is 

 attached to it and so is in contact with it. This 

 enables Nature to use any couple of bones either 

 as a single connected piece, or, when flexion is 

 required, as two distinct pieces. In like manner, 

 there is no such thing as a blood-vessel by itself in 

 isolation : they are all of them parts of one blood- 

 vessel. An isolated bone could never discharge the 

 function for which all bones exist ; for, being dis- 

 continuous and disconnected from the rest, it could 

 never sei've as the means either for bending or for 

 straightening a limb ; but Avorse than that, it would 

 be a source of harm, like a thorn or an arrow sticking 

 in the flesh. Similarly, if we imagine a blood-vessel 

 isolated and not connected with the source of them 

 all, it could never keep the blood within it in a proper 

 condition, since it is the heat Λvhich comes from that 

 source which prevents the blood from congeahng, as 

 is shoAvn by the putrefaction of blood when separ- 

 ated from it. This source of the blood-vessels 

 is of course the heart, and the corresponding source 

 of the bones in all bony species is what is called the 

 backbone. The system of the bones is a connected 

 whole, starting from the backbone, since the back- 

 bone connects together the length of the animal's 

 body and holds it straight. Now although this back- 

 bone is a unity because it is connected together, it 



f2 163 



