THE DENTAL TISSUES. 99 



in that thin layer which invests the front of the enamel of 

 the rodent-like tooth of a wombat. 



The cementum at the neck is also devoid of lamellae ; it 

 appears to be built up by direct ossification of osteoblasts, 

 the prismatic shape of which may be traced in it : Bodecker 

 describes it as permeated by a fine but abundant network 

 of soft living matter. The larger dentinal tubes fall short 

 of the boundary line at the neck, but a fine protoplasmic- 

 network crosses it. Bodecker states that it has a covering 

 of epithelial elements, like those of the gum. 



The outermost layer of thick cementum is a glassy film,, 

 denser apparently than the subjacent portions, and quite 

 devoid of lacunae ; on the surface it is slightly nodular, and 

 might be described as built up of an infinite number of 

 very minute and perfectly fused globules ; this is, in fact, 

 the youngest layer of cement, and is closely similar to that 

 globular formation which characterizes dentine at an early 

 stage of its development. 



The cementum is very closely, indeed inseparably, con- 

 nected with the dentine, through the medium of the 

 " granular " layer of the latter ; the fusion of the two tissues 

 being so intimate, that it is often difficult to say precisely 

 at what point the one may be said to have merged into the 

 other. And in this region there is an abundant passage of 

 protoplasmic filaments across from the one to the other. 



Nasmyth's membrane. Under the names of Nasmyth r s 

 membrane, enamel cuticle, or persistent dental capsule, a 

 structure is described about which much difference of 

 opinion has been, and indeed still is, expressed. Over the 

 enamel of the crown of a human or other mammalian tooth, 

 the crown of which is not coated by a thick layer of cemen- 

 tum, there is an exceedingly thin membrane, the existence 

 of which can only be demonstrated by the use of acids, 

 which cause it to become detached from the surface of the 

 enamel. When thus isolated it is found to form a continu- 



n 2 



