LESSON 53.] THE STRUCTURE OP THE TEETH. 189 



gist of three distinct tissues ; the dentine, or ivory, which forms the 

 greater part of the tooth, and contributes to its form ; the enamel, 

 which covers and defends the outer, exposed surface of the tooth ; 

 and lastly the crusta petrosa, or cementum, which forms the external 

 covering of the fang. The interior of a tooth contains a cavity of 

 variable size, called the pulp cavity, in which, in a healthy tooth, the 

 vascular pulp, with its plexus of nerves, lies. 



813. As the enamel first meets our gaze, it shall have priority of 

 description. This tissue, in the grinding teeth, is thickest on the 

 masticating surface. A delicate membrane, discovered by Nasmyth, 

 and known as " Nasmyth's membrane," covers the enamel, and is so 

 closely united with it, that they can only be separated by the em- 

 ployment of hydrochloric (formerly muriatic) acid. 



814. The enamel is so much more brittle, and harder than the 

 other tissues of the teeth, that a knife scarcely marks it, and it 

 readily yields sparks when struck against steel. 



815. This tissue presents a fibrous structure, due to its compo- 

 sition, which consists entirely of the prisms or fibres of the enamel. 

 These are long, solid prisms, irregular in shape, but usually hexa- 

 gonal, or pentagonal, generally occupying the whole substance of the 

 enamel, resting by one extremity on the dentine (ivory), and by the 

 other on Nasmyth's membrane. 



816. In adult teeth it is easy to detect these elements in longi- 

 tudinal or transverse sections, but it is difficult to separate them for 

 any great length ; it is easier of accomplishment in young and devel- 

 oping teeth, where the enamel is so much softer that it may be 

 cut with a knife. The action of diluted hydrochloric acid is 

 necessary to effect the separation of the prisms, and, if not too 

 long continued, delicate transverse striae may be detected on their 

 surface. 



817. The prisms of the enamel are very firmly united, but what 

 constitutes the bond of union, has not yet been satisfactorily demon- 

 strated; delicate tubes have been clearly established in connection 

 with this tissue in young animals, and although difficult of detection 

 in adults, the probability is that they are present, although it may 

 be in a calcified condition. 



818. In making a section of a tooth, to show the facets of the 

 prisms of enamel, great care should be taken not to injure the ex- 

 ternal surface, but grind from behind until sufficiently thin. 



819. A figure of such a section, from a human tooth so prepared, 



