172 ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



with concentric laminae, and in the absence of the characteristic lacunae and 

 canaliculi. Irregular cavities, however, are scattered through them ; which 

 may in some degree answer the same purpose. The osseous plates not un- 

 frequently found in the dura mater, are stated by Mr. Tomes to possess a 

 structure more closely allied to that of true bone ; which may be connected 

 with the fact that, in some of the lower Mammalia, certain parts of this mem- 

 brane (the falx and tentorium) are normally ossified. 



204. The Regeneration of Bone, after loss of its substance by disease or 

 injury, is extremely complete ; in fact, there is no other structure of so com- 

 plex a nature, which is capable of being so thoroughly repaired. Much dis- 

 cussion has taken place with respect to the degree in which the different 

 membranous structures, that surround bone and penetrate its substance, con- 

 tribute to its regeneration ; but the fact seems to be, that any or all these 

 membranes may contribute to the formation of new bone, in proportion to 

 their vascularity, the new structure, however, being most readily produced 

 in continuity with the old. Thus, when a portion of the shaft of the bone 

 is entirely removed, but the periosteum is left, the space is filled up with 

 bony matter in the course of a few weeks ; though, if the periosteum also be 

 removed, the formation of new osseous matter will be confined to a small 

 addition in a conical form to the two extremities, a large interspace being left 

 between them. The production of new bony tissue, in this experiment, as 

 in cases where the periosteum has been detached by disease and remains alive 

 while the shaft dies, is in continuity with minute spicula of original bone, 

 which still adhere to the membrane ; and it is well known that, in comminuted 

 fractures, every portion of the shattered bone, that remains connected with 

 the vascular membranes, whether these be internal or external, becomes the 

 centre of a new formation ; the loss of substance being filled up the more ra- 

 pidly? in proportion to the number of such centres. 



205. The most extensive reparation is seen, when the shaft of a long bone 

 is destroyed by disease. If violent inflammation occur in its tissue, the death 

 of the fabric is frequently the consequence ; apparently through the blocking- 

 up of the canals with the products of inflammatory action, and the consequent 

 cessation of the supply of nutriment. It is not often that the whole thickness 

 of the bone becomes necrosed at once ; more commonly this result is confined 

 to its outer or to its inner layers. When this is the case, the new formation 

 takes place from the part that remains sound ; the external layers, which 

 receive their vascular supply from the periosteum, and from the Haversian. 

 canals continued inwards from it, throwing out new matter on their interior, 

 which is gradually converted into bone ; whilst the internal layers, if they 

 should be the parts remaining uninjured, do the same on their exterior, de- 

 riving their materials from the medullary membrane, and from its prolonga- 

 tions into their Haversian canals. But it sometimes happens that the whole 

 shaft suffers necrosis; and as the medullary membrane and the entire Haver- 

 sian system have lost their vitality, reparation can then only take place from 

 the splinters of bone which may remain attached to the periosteum, and from 

 the living bone at the two extremities. This is consequently a very slow- 

 process ; more especially as the epiphyses, having been originally formed as 

 distinct parts from the shaft, do not seem able to contribute much to the re- 

 generation of the latter. 



206. When the shaft of a long bone has been fractured through, and the 

 extremities have been brought evenly together, it is found that the new matter 

 first ossified is that which occupies the central portion of the deposit, and 

 which thus connects the medullary cavities of the broken ends, forming a kind 

 of plug that enters each. This was termed by Dupuytren, by whom it was 

 first distinctly described, the provisional callus, and it is usually formed in 



