72 MICROSCOPIC ANATOMY OF THE TEETH 



teeth, from the alternate bleaching and pigmentation of the 

 enamel seen in this affection. This pathological condition 

 affects only the interprismatic substance and not the 

 prisms. The work of calcification appears to be reversed, 

 and the last-formed part of the enamel, the interprismatic 

 substance, is removed by the action of some as yet un- 

 known agent which has no effect upon the prisms. This 

 remarkable complaint, investigated by such an accurate 



FIG. 30. Striae of Retzius. ( x 150.) 



observer as Dr. Black on the spot, and by means of a very 

 carefully prepared series of microscopical sections, lends 

 the greatest confirmation to the view that the prisms and 

 the interprismatic substance are separate structures. The 

 question then arises, how are they distinct from one another ? 

 Is the cement substance only a persistently less completely 

 calcified element of the enamel or is it in some way of 

 a different composition ? 



The so-called ' brown strise of Retzius ' (figs. 30, 31, 32) are 

 brown lines which conform to the position of the enamel cusps 

 at different stages of growth and are probably marks of their 

 stratification, an opinion held by Kolliker, Walkhoff, and 

 others. 



