ENAMEL OF THE TEETH. 99 



however, much less striking in human teeth, so that the que - 



tion as to which of the two views is correct must remain un- 

 decided. 



What, tlieu, is the process of formation of these enamel- 

 prisms ? According to Purkinje and Raschkow, the crown of the 

 growing tooth is surrounded externally by a peculiar membrane, 

 the enamel-membrane, the inner surface of which is composed 

 of short hexagonal fibres, which stand perpendicularly upon the 

 membrane, and are directed towards the enamel, so that each 

 fibre of the enamel-membrane corresponds to an enamel-fibre. 

 On examining a portion of this membrane, particularly that 

 part which lies nearest to the root of the tooth, we readily 

 recognize in it the characteristic nuclei, some of them being 

 furnished with nucleoli. They lie in a minutely granulous 

 substance. This granulous aspect, however, is seen to be pro- 

 duced, in many situations, by granulated cells which contain the 

 nuclei. Each nucleus is surrounded by a circular areola of 

 small granules, and seems to lie in a minutely granulated 

 globule, which we know to be the rudimentary form of 

 most elementary cells. Some of these cells are prolonged 

 into very delicate fibres; they appear to be young cells of 

 areolar tissue; most of them, however, are round. The 

 fibres or prisms of the membrane, which have a direction 

 from its inner surface towards the enamel-fibres, have 

 assumed an hexagonal form, which Raschkow attributes to 

 their close contact. They very closely resemble the columnar 

 epithelium upon mucous membranes, only that they arc 

 prismatic in their entire length, that is, so far as they project 

 out from the membrane to which they are attached. I am 

 inclined therefore, to regard them as merely elongated cells. 

 In the recent state they also contain a very distinct nucleus, 

 which encloses its nucleolus. (See pi. Ill, fig. 4.) In the upper 

 part of the enamel-membrane they lie quite close together ; 

 but in the portion nearest to the root of the tooth, they 

 diminish in number and stand insulated, so that at this 

 part the structure of the membrane beneath them may 

 also be recognized, and I suppose the round cells before 

 mentioned to be the earlier condition of these prismatic 

 cells. What, then, is the relation which these prismatic cells 

 of the enamel-membrane bear to the prisms of the enamel? 



