THE OSSEOUS SYSTEM. 381 



which, as I have before endeavoured to show, is still retained in 

 every lacuna, are most indispensably requisite for the main- 

 tenance of the bone; for we see, that when the supply of blood 

 to a bone is impeded by the destruction of the periosteum or of 

 the medulla, by ligature of the vessels of the limb, or by oblitera- 

 tion of the periosteal vessels by pressure from without (aneurism, 

 tumours), necrosis of the parts involved certainly ensues, and 

 can scarcely in any case be altogether obviated by the collateral 

 circulation which actually exists also in the bones (vid. supra). 

 On the other hand we are scarcely, at present, in a condition to 

 say, how the circulation of the plasma in the bones is carried 

 on, though its movement to and from vessels (perhaps from the 

 arterial, through several lamellar systems to the venous) must 

 probably be assumed ; or what special changes in the course of 

 the nutrition of bone take place; with the latter in particular 

 we are unacquainted, because the chemical investigation of 

 these changes, and especially of the organic products of decom- 

 position, is still altogether imperfect. 



That the osseous tissue is in a state of constant, and indeed 

 very energetic molecular change, is evidenced not only in its 

 various morbid conditions, but also bv the alterations it 

 undergoes in old age. These alterations consist more especially 

 in the disappearance of entire portions of the bones, either 

 externally or internally; of the former, an instance is afforded, 

 in the entire removal of the alveolar processes of the jaws, and 

 the latter is seen in the greater porosity and fragility of every 

 kind of bone, such as the cylindrical bones and those of the 

 cranium, in the enlargement of the vascular openings (vertebra, 

 apophyses), and in the greater roughness of the surfaces of the 

 bones. This senile atrophy of the bones may also be attended 

 consecutively with an internal addition of bone-substance, a 

 sclerosis as it is termed, as in the flat bones of the cranium, in 

 consequence of which, in direct contrast to the phenomena 

 elsewhere presented by senile bone, the diplo'e disappears, its 

 cancelli becoming filled up by new osseous tissue, whilst the 

 venous spaces and foramina emissaria are obliterated and the 

 entire bone rendered heavier. 



With this abundant vascular supply, and certainly not 

 sluggish molecular change, it cannot be surprising that the 

 bones should be so richly furnished with nerves, the principal 



