HISTOLOGY OF THE HARD DENTAL TISSUES. 119 



la some specimens which I will place under my microscope this 

 evening, several degrees of growth of this secondary dentine may 

 be observed, from a slight patch on the walls of the pulp-cavity, to 

 the entire obliteration of the cavity by this new formation. But I 

 must hasten on to the brief consideration of the last of the hard 

 dental tissues which will engage our attention this evening, namely, 

 that which forms the clothing of the fang, and assists in its attach- 

 ment to the jaw. 



Referring again to our longitudinal section, we shall see the 

 enamel on the crown thickest at its apex, and gradually diminishing 

 at the neck of the tooth, but where it vanishes another substance 

 will be seen commencing, at first in a very thin layer, which enlarges 

 as it progresses down the sides of the tooth, reaching its maximum 

 of thickness about the apex of the fang. In its histological cha- 

 racter, it approximates more closely to true bone than does any 

 of the other dental tissues, its development and character being 

 allied to that of the bones generally. 



Arising from the fibrous remains of the sac originally investing 

 the primordial tooth, numerous processes shoot inwards ; as ossifi- 

 cation proceeds, these processes unite at different points, enclosing 

 interspaces ; round the peripheral portions of these spaces the new 

 bone is formed in an irregular deposition, and after a time this pro- 

 cess stops, and lacunae having radiating canaliculi remain, being filled 

 with a soft protoplasmic substance. This, then, is the general 

 character of the investing substance coating the roots of the teeth. 

 This cementum, as it is termed, like true bone, is permeable to the 

 nutrient fluids of the body through the periosteal membrane, by 

 which it is connected with the jaw ; and it is not difficult, with high 

 powers, to demonstrate fine tubuli which, entering in its periosteal 

 surface, form channels of communication through the lacunae and 

 canaliculi with the dentine, so that when the " nerve " of a tooth 

 perishes, a partial vitality is kept up through the medium of the 

 cementum ; but this tissue is likewise the seat of pathological 

 changes which ought to be known to all who may examine the 

 sections usually met with — I refer to that especially known as 

 " exostosisy When much irritation from any cause arises around 

 the root of a tooth, and inflammation sets in, thjs tissue becomes 

 sometimes enormously hypertrophied, almost every exacerbation of 

 the attack being marked by a fresh stratification of the osseous 

 tissue, while the lacunae are much larger and more irregular in 



