154 ADAPTATION OF BON^S. 



ness and vigour of respiration, the lungs enlarge, and, 

 to give them scope, the chest br omes full, broad, 

 and capacious, in old age, again, when the season 

 of active exertion is over, and the strength decays, 

 the broad shoulders and capacious chest of man- 

 hood gradually disappear, and a totally different 

 form occupies its place. Now, at all these periods, 

 the bones are the parts which, by their alteration, 

 serve as an index of the changes going on within ; 

 and on this large scale, the difference in their form 

 is so great that it must be obvious to all. 



Where the whole of the soft contents of the bony 

 cavity increase in size, as happens in the case of 

 water in the head, the result is, as already mentioned, 

 an expansion from interstitial growth of the osseous 

 covering. But where the tumour or pressure is 

 limited to a small part, a process of a different kind 

 often takes place, which has also the preservation of 

 life for its object, and which is accomplished by another 

 of the natural actions, absorption. When a bone, 

 say of four inches square, is required gradually to ex- 

 pand itself, so as to protect a surface of six inches 

 or of double the extent, this is accomplished by the 

 gradual removal of the old, and the deposition of 

 new and additional particles, on, as it were, a new 

 and enlarged mould. But in the other case, where 

 the pressure is very limited where, for instance, a 

 small tumour develops itself on the surface of the 

 brain, which, if allowed to grow within unyielding 

 walls, would soon cause death by pressure on the 

 brain the ordinary process of absorption becomes 

 greatly excited, and gradually eats away the whole 

 thickness of the bone over the tumour, which then 

 protrudes externally, and relieves the brain within 

 from pressure which would have been fatal to it. 



I have already stated, that besides a large pro- 

 portion of earthy matter, which gives to them dry- 

 ness and hardness, bones contain a large quantity 

 of animal matter, which is essential to their constitu- 



