THE OSSEOUS SYSTEM. 339 



periosteum or of the medulla, by ligature of the vessels of the limb, or 

 by obliteration of the periosteal vessels by pressure from without 

 (aneurism, tumors), necrosis of the parts involved certainly ensues, and 

 can scarcely in any case be altogether obviated by the collateral circula- 

 tion which actually exists also in the bones (vid. sujjra). On the other 

 hand we are scarcely, at present, in a condition to say, how the circula- 

 tion 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 par- 

 ticular we are unacquainted, because the chemical investigation of these 

 changes, and especially of the organic products of decomposition, 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 by the alterations it undergoes in old age. These 

 alterations consist more especially in the disappearance of entire por- 

 tions 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 (vertebrce, apo- 

 physes), 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 diploe 

 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 mole- 

 cular change, it cannot be surprising that the bones should be so richly 

 furni:<hed with nerves, the principal function of which appears to me to 

 consist in the regulation of the conditions of the vascular system, by 

 their conveying to the central organ (spinal cord) through the sensitive 

 fibres intelligence of the state of the vessels, of the quantity of nutri- 

 tive fluid in the bone, and probably also of the modus of the molecular 

 change going on in themselves, and by means of the motor elements 

 their bringing a reflex influence from it, to the arteries and veins which 

 are manifestly furnished with contractile fibres. These unconscious and 

 involuntary alternations of influence of sensible and motor filaments, 

 are, as it appears to me, the most important phenomena of the innerva- 

 tion in bones, as well as in all other orfjans, the nerves of which are 

 not constantly in relation with the external world, and make it intelli- 

 gible, why it is, that no organ, containing nerves and vessels at all, . 



