THE DENTAL TISSUES. 



45 



however, subject to many minor modifications. The curved 

 and decussating course of the human enamel prisms renders 

 them difficult to trace throughout their length, but the 

 structure of the enamel of many lower animals (especially 

 the rodents) is more easily intelligible. Enamel such as 

 that of the Manatee, in which all the prisms pursue a 

 perfectly straight course, is of comparatively rare occurrence, 

 but among the rodents the courses pursued by the enamel 

 prisms are simple, and produce very regular patterns, which 

 are constant for particular families (J. Tomes). Thus, in 

 the Sciuridce, a section of the enamel, whether longitudinal 



FIG. 18 (*). 



or transverse, appears divided into an outer and inner 

 portion, in which the prisms, although continuous from the 

 dentine to the free surface, pursue a different direction. As 

 seen in the longitudinal section, the enamel prisms start 

 from the dentine at right angles to its surface, and after 

 passing through about two-thirds of the thickness of the 

 enamel in this direction, abruptly bend upwards at an angle 

 of 45 degrees with their original course. In transverse 

 section the enamel prisms are found to be arranged in hori- 



(*) Section of deutine and enamel of a Beaver : in the inner half the 

 prisme of contiguous layers cross each other at right angles, in the outer 

 they are parallel. 



