382 GENERAL ANATOMY. 



terior by addition, and diminished at the interior by abstrac- 

 tion, whence resulted the enlargement of the canal. 



In fact, when the growth of the bone in thickness is accom- 

 plished, the canal continuing to enlarge by internal absorption, 

 its walls become thin in a singular degree, insomuch that, 

 after having been thicker in the child than the diameter of the 

 canal, and in the adult nearly as thick, they present in old age 

 but a very small fraction of that diameter. The spongy cavi- 

 ties of the short bones, of the broad bones, and of the extremi- 

 ties of the long bones, generally enlarge in the same manner, 

 so that, by this diminution of the substance of the bones, the 

 skeleton of aged persons is rendered much lighter than that of 

 others. 



The broad bones of the skull pretty frequently undergo a 

 diminution in thickness of another kind in old age. It results 

 from the absorption of the diploe, and the approximation of 

 the outer table to the inner, so as to produce at the same time 

 a great diminution of thickness and an external depression. 

 It is in the parietal prominences which are frequently affected 

 by it, that this wasting generally commences. 



Frequently also, in old age, the articular surfaces of the 

 bones of the inferior members and the faces of the vertebrae 

 are widened and flattened, as if they had at length yielded to 

 pressure. 



604. The form of the bones is not the only property that 

 undergoes changes from the advance of age. Their consistence 

 also exhibits remarkable changes; the bones of children are 

 more flexible and less brittle than those of adults, and may be 

 bent or twisted in the living subject without breaking. Those 

 of old persons, on the contrary, are denser, harder, and more 

 brittle than those of adults, which circumstances, added to 

 their having become thinner, renders fractures very common 

 in old age. There is also a sensible difference in the proportion 

 of the earthy substance, it being greater in old age than in the 

 adult state. 



Thus, after the growth in dimensions has terminated, the in- 

 crease of the density continues in the bones, as in all the other 

 parts of the body. 



