

FORMATION OF TEETH. 185 



medullary cavities of the broken ends, forming a kind of plug that 

 enters each. This was termed by Dupuytren, by whom it was first dis- 

 tinctly described, the provisional callus. This is usually formed in the 

 course of five or six weeks, or less in young subjects ; but at that period 

 the contiguous surfaces of the bone itself are not cemented by bony 

 union ; and the formation of the permanent callus occupies some months, 

 during which the provisional callus is gradually absorbed, and the con- 

 tinuity of the medullary canal restored, in the same manner as it was 

 at first established. The permanent callus has all the characters of true 

 bone. It seems to have been established by the observations of Mr. 

 Paget, however, that these statements do not usually apply to the case 

 of Man ; in whom, when the limb is kept at rest, the union between the 

 fractured ends is accomplished by ossification of the substance connect- 

 ing them, without the intermediation of a provisional callus ; this being 

 only formed when the portions of the bone are kept in continual move- 

 ment. 



309. 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 the inflam- 

 matory 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 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 vas- 

 cular supply from the periosteum and from the Haversian canals con- 

 tinued 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, 

 deriving their materials from the medullary membrane and its prolon- 

 gations into their Haversian canals. But it sometimes happens that 

 the whole shaft suffers necrosis ; and as the medullary membrane and 

 the entire system of Haversian canals have lost their vitality, reparation 

 can only take place from the periosteuum, 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 regene- 

 ration of the latter. 



310. We next proceed to the Teeth, which are, organs of mechanical 

 attrition, developed in the first part of the alimentary canal, for the 

 purpose of comminuting the food conveyed into it. Their place of ori- 

 gin is altogether different from that of bone, as they commence in little 

 papillary elevations of the mucous membrane covering the jaw ; but the 

 substance from which they are formed is the same primitive cellular tissue, 

 as that in which Cartilage itself originates. We may best understand 

 the structure and development of the Teeth in Man, by first inquiring 

 into the characters presented by those of some of the lower animals, 

 and the history of their evolution. In the foetal Shark, the first appear- 

 ance of the tooth is in the form of a minute papilla on the mucous mem- 

 brane covering the jaws ; the substance of this papilla is composed of 

 spherical cells, which are imbedded in a kind of gelatinous substance 



