XX INTRODUCTION. 



concentric striae, of which some are more clearly and others more 

 faintly visible, as if the cement were deposited in fine and coherent 

 layers. The layer of cement is found in the deciduous teeth, but 

 is relatively thinner and the Purkinjian cells are more irregular. 



" In growing teeth with fangs not fully formed, the cement is 

 so thin that the Purkinjian cells are not visible : it looks like a fine 

 membrane, and has been described as the periosteum of the fangs, 

 but it increases in thickness with the age of the tooth, and is the 

 seat and origin of what are called exostoses of the fang which are 

 wholly composed of it." These growths are subject to the formation 

 of abscess, and all the other morbid actions of true bone. 



It is the presence of this osseous substance which renders 

 intelligible many well-known experiments of which human teeth 

 have been the subjects ; such as their transplantation and adhesion 

 into the combs of cocks, and the establishment of a vascular con- 

 nection between the tooth and the comb ; the appearances which the 

 Hunterian specimens of these experiments present, and of the reality 

 of which Professor MuUer satisfied himself during his visit to 

 London, are no longer perplexing, now that we know that the surface 

 of the tooth, in contact with, and adhering to the vasqular comb, 

 is composed of a well organised tissue, closely resembling bone. 



This correspondence of the cement, which, when it exists in 

 sufficient quantity, becomes almost identity, with true bone, is illus- 

 trated by the varieties of microscopic structure which the cement 

 presents in different classes of animals, and which always correspond 

 with the modifications of the osseous tissue of the skeleton in those 

 animals ; thus the cement in the osseous fishes, in which the bone 

 is not characterized by the radiated calcigerous cells, likewise ceases 

 to present that character ; and, in reptiles and mammals in which 



