STRUCTURE. 13 



pharyngeal denticles of the scari, and the lamelliform denticles of 

 the crop-fishes, diodon and tetrodon, likewise the maxillary teeth of 

 some of the genera of sharks and rays, afford examples of this structure. 



I have spoken of it as belonging to a higher type because it is 

 characteristic of the teeth of the vertebrate animals which are higher 

 in the scale than fishes ; but if the grade of organization of a tooth 

 be rated according to the proportion of vascular substance and vital 

 power which it possesses, then those teeth which most resemble bone 

 should be regarded as the most highly organized, and such are the 

 teeth most common in the class of fishes. 



As the inorganic calcareous particles are deposited principally 

 in the parietes of the calcigerous tubes and their terminal ramuli 

 and cells, it follows that the density of the tooth will increase, and 

 its vital properties diminish as the calcigerous preponderate over the 

 medullary tubes, and in proportion as the calcigerous tubes in a given 

 space are more numerous and minute. But the change from one 

 modification of dental structure to another, from bone to the densest 

 ivory, is so gradual, that the physiologist, entertaining a belief in the 

 inorganic nature of the teeth, would be at a loss where to draw 

 the line, or to determine where the vital forces ceased their manifesta- 

 tion, and at which step in the series of tubular structures the 

 tooth became an inert body. The uniform result of my researches, 

 on the structure of the teeth in all grades of vertebrate animals, 

 and in their natural and diseased states, has been a conviction of 

 the untruthfulness of the terms inert, dead, and unorganized as 

 applied to the substance of any tooth whatever. Extra-vascular 

 undoubtedly is all that portion which consists of the calcigerous 

 tubes ; the capillary circulation is confined to the pulp or medul- 

 lary canals ; but since every secretive process and the development 

 of the primordial cells of every tissue are due to changes produced 

 in the liquor sanguinis transuded from and beyond the sphere of 

 the ultimate capillaries, the absence of these vessels in the dense 

 dental substance is as little conclusive against its vital and organized 

 nature, as it would be to prove the inert condition of the germinal 

 membrane of the ovum before the thirtieth hour of incubation. 



In no teeth is the dentine rendered so dense as in those of certain 



