46 A MANUAL OF DENTAL ANATOMY. 



zontal layers, each layer being a single fibre in thickness ; 

 in alternate layers the prisms pass to the right and to the 

 left, crossing those of the next layer at right angles, and 

 thus making a pattern of squares in the inner two-thirds of 

 the enamel. But in the outer third of the enamel, where 

 the prisms bend abruptly upwards, those of superimposed 

 layers no longer pass in opposite directions, but are all 

 parallel ; in fact no longer admit of distinction into laminae. 



Thus each enamel prism passes in a very definite direction 

 and, seen with those of other layers, forms a very charac- 

 teristic pattern ; but the enamel prisms are not in any part 

 of their course curved. 



In the beaver, from which the foregoing figure is taken, 

 the arrangement of the enamel prisms is dissimilar in the 

 upper and lower teeth, the lamination taking place in 

 different directions, so that a longitudinal section of the one 

 might, so far as this is concerned, be mistaken for a trans- 

 verse section of the other. As regards the decussation of 

 the prisms of alternate layers, it is similar to that of the 

 Sciuridce, but it differs in the laminae being slightly flexuous 

 instead of pursuing perfectly straight lines. 



Among the porcupine family very much more complex 

 patterns are met with, the enamel prisms being individually 

 flexuous, and their curves not being confined to one plane ; 

 the individual prisms pursue a serpentine course, and cannot 

 be followed far in any one section. Near to the surface, 

 however, they all become parallel, the enamel thus conform- 

 ing with that of other rodents in being divided into two 

 portions (at least so far as the course pursued by, and the 

 pattern traced by, its fibres in its inner and outer parts can 

 be said to so divide it). The Leporidce, or hares, form an 

 exception to this ; their enamel has no such lamelliform 

 arrangement, but is built up merely of slightly flexuous 

 prisms. 



By tracing the courses of enamel prisms from the simple 



