478 



SPECIAL HISTOLOGY. 



Fig. 189. 



Fijr. 190. 



of 0-0015-0*0022 of a line in breadth, irregular in shape, but commonly 

 hexagonal or pentagonal, which usually occupy the 

 whole thickness of the enamel, resting with one ex- 

 tremity upon the dentine, and with the other upon 

 Nasmyth's membrane. In adult teeth, these elements 

 may be very readily detected in transverse and longi- 

 tudinal sections, but can hardly be isolated for any 

 great length; it is otherwise in young or developing 

 teeth, where the enamel is much softer and may be 

 cut with a knife. In such isolated prisms, whose broken ends may by 

 accident appear pointed, whence they have been called " enamel 

 needles," the surfaces and edges may be very well seen. We may also 

 very frequently observe upon them, particularly after the addition of 

 dilute hvdrochloric acid, more or less distinct transverse striae 0-0014- 

 0-002 of a line apart, which arise from slight varicosities, and give the 

 fibres a certain resemblance to muscular bundles, or rather colossal mus- 

 cular fibrils. They certainly do not 

 indicate a cellular composition. If 

 the action of the hydrochloric acid 

 be allowed to go on, the fibres soon 

 become quite pale, the transverse 

 striation disappears, and nothing re- 

 mains but a delicate framework cor- 

 responding with the previously solid 

 fibres, and which often presents cer- 

 tain appearances of tubes. In the 

 end this also becomes almost com- 

 pletely destroyed by the action of 

 the acid, so that in teeth which have 

 been thus macerated hardly any- 

 thing remains of the enamel, which 

 does not, like the dentine, retain its form. 



The prisms of the enamel are uJiited very intricately without any in- 

 termediate substance. I have not been able to convince myself that 

 canals constantly exist between the prisms,* but it is certain that cavi- 



Fio. 189. — Surface of the enamel, with the ends of the enamel prisms, magnified 350 

 diameters. From the Calf. 



Fig. 190. — Enamel prisms isolated, after the slight action of hydrochloric acid; magni- 

 fied 350 diameters. From Man. 



* [With respect to this point, opinions differ ; Todd and Bowman consider that canals 

 normally exist between the enamel prisms. Tomes finds canals in the enamel prisms of 

 young animals, and sometimes in a part or the whole length of them in old teeth. Kolliker 

 (Mikr. Anat. 77) has not yet observed any such cases. Czermak (I.e. p. 13) believes that, 

 in a few cases, he has observed " very numerous delicate enamel tubules arranged in close 

 series." — Trs.] 



