710 THE MICROSCOPE 



preformativa" of the whole pulp. Nasmyth says : " In all 

 cases where this covering has been removed by means of 

 acid, it has, of course, the appearance of a simple mem- 

 brane, in consequence of the earthly deposit having been 

 dissolved out, and the animal tissue only remaining. The 

 structure and appearance of the covering detached in this 

 manner from the enamel is the same in every respect as 

 that observed in the capsule of the unextruded tooth, and 

 consisting, like it, of two layers, fibrous externally, and 

 having on its external surface the peculiar reticulated 

 appearance common to both." 



" On examining carefully fine sections of several teeth 

 under the microscope, I perceived here also," observes 

 Nasmyth, " that the structure in question was continuous 

 with the crusta petrosa of the fang of the tooth." 



The enamel has a fibrous bluish aspect, is very brittle, 

 and much harder than the other dentinal structures ; it 

 is, indeed, so hard, that it strikes fire with steel ; if an 

 attempt be made to cut it without the application of water 

 to keep it cooled down, it burns with an ammoniacal 

 odour, such as we perceive when horse-hoof is burnt. It is 

 composed of prisms, about l-5,000th of an inch in breadth, 

 more or less wavy, and transversely striped. Two kinds of 

 bands or stripes are seen traversing enamel, the direction 

 of one of which nearly coincides with that of the dentine 

 fibres ; the other set of stripes indicate the laminated struc- 

 ture of enamel. Under polarised light, a third set become 

 visible, arising from the variable inclination of the axes 

 of the fibres to the plane of polarisation. The enamel 

 is often traversed by cracks or fissures, mostly running 

 parallel with one set of the fibres : these are sometimes 

 described as canals ; but as they resemble splits, and are 

 seldom seen in young teeth, it is more likely that they 

 are caused by the nature of the food and drink, which is 

 taken into the mouth at temperatures varying many de- 

 grees ; we also trace the commencement of disease from 

 a fissure in the enamel. When a section of the enamel 

 is cut obliquely, it has somewhat of a hexagonal or six- 

 sided appearance. The dentine consists of a transparent 

 basement membrane, with alternating layers of cal- 

 careous matter, traversed by very fine branching tubuli, 



