62 A MANUAL OF DENTAL ANATOMY. 



course is sometimes apparently interrupted by a small inter- 

 globular space, as is to be seen in an extreme degree in the 

 dentine of Cetacea. 



Owing to their breaking up into minute branches, some 

 of the tubes become lost as they approach the surface of the 

 dentine, and apparently end in fine pointed extremities. 



Some terminate by anastomosing with terminal branches 

 of others, forming loops near to the surface of the dentine ; 

 others terminate far beneath the surface in a similar way. 



Some tubes pass into the small interglobular spaces 

 which constitute the "granular layer" described by my 

 father, while others again pass out altogether beyond the 

 boundary of the dentine and anastomose with the canaliculi 

 of the lacunse in the cementum. 



The enamel also may be penetrated by the dentinal 

 tubes, though this when occurring in the human subject 

 must be regarded as exceptional and almost pathological 

 in its nature (see Fig. 22). As has, however, been men- 

 tioned in speaking of the enamel, in most of the Marsupials 

 and in certain other animals it is a perfectly normal and 

 indeed characteristic occurrence, difficult though it be to 

 see how such a relation of parts is brought about in the 

 course of development of the two tissues. 



Dentinal Sheaths. If dentine be exposed to the action 

 of strong acid for some days, a sort of fibrous felt, or if the 

 action of the acid has gone further, a transparent slime alone 

 remains. Examined with the microscope this proves to be a 

 collection of tubes ; it is, in fact, made up of the immediate 

 walls of the dentinal tubes, the intervening matrix having 

 been wholly destroyed. 



Two facts are thus demonstrated : the one that the tubes 

 have definite walls, and are not simple channels in the 

 matrix ; the other, that these walls are composed of some- 

 thing singularly indestructible. Indeed, the walls of the 

 dentinal tubes are so indestructible that they may be 



