STRUCTURE OF THE CEMENT. 



791 



It is also found, on treatment with acid, that a very thin membrane 

 called by Kolliker "cuticle of the enamel," and by Busk and Huxley 

 " Nasmyth's membrane, " (after its discoverer), entirely covers the enamel upon 

 its outer surface. This membrane, which is calcined in the natural state, 

 forms a protective covering to the enamel. Berzelius and Retzius say that 

 a similar membrane also exists between the enamel and the dentine, but 

 Kolliker has been unable to find any in that situation. 



c. The crusta petrosa or cement is the third substance which enters into 

 the formation of the teeth. This is a layer of true bone, slightly modified 

 in structure, and investing that part of the dentine which is not protected 

 by the enamel. It covers the whole fang, towards the lower end of which 

 it becomes gradually thicker, and is especially developed at the apex, and 

 along the grooves of the compound fangs. Besides this, the calcified mem- 

 brane or cuticle on the surface of the enamel has been regarded by various 

 writers as a coating of cement iu that situation, the representative of the 

 coronal cement on the compound teeth of many herbivorous animals. As 

 life advances, the cement generally becomes thicker, especially near the 

 apex of the fang, where it sometimes blocks up the orifice leading into the 

 pulp-cavity. 



The crusta petrosa contains cells and canaliculi resembling those of bone ; 

 they are placed lengthwise around the fang, and give off minute radiated 

 ramifications, which are often found to proceed from one side only of a cell, 

 towards the periodontal surface (Tomes). In the deeper layers of the 

 cement the fine canaliculi sometimes anastomose with some of the terminal 

 tubules of the subjacent dentine. Where the cement is very thick it may 

 contain vascular canals, analogous to the Haversian canals of bone. On the 

 deciduous teeth the cement is thinner, and contains fewer cells. It has been 

 shown by Sharpey that perforating fibres, similar to those of ordinary bone, 

 run abundantly through the cement. In chemical composition it resembles 

 bone, and contains 30 per cent, of animal matter. The cement is, according 

 to some, extremely sensitive at the neck of the tooth, if it be exposed by 



Fig. 550. SECTION OP A PORTION OP THE DENTINE AND CKMENT FROM THE MIDDLE OF 

 THE ROOT OP AN INCISOR TOOTH (from Kolliker). 3 f 



a, dental tubuli ramifying and terminating, some of them in the interglobular spaces 

 (b and c), which resemble somewhat bone-laeunse ; d, inner layer of the cement witl\ 

 numerous closely set canaliculi ; e, outer layer of cement; /, lacunce ; y, canaliculi. 



