TEETH. 569 



two ways, these are mere varieties of the same mode in 

 the animal kingdom. In the first, which may be typified 

 by the Mackerel and the Frog, the pulp is never free, but 

 from the first is inclosed within the capsule, seeming to 

 sink down as fast as it grows. In the other, the pulp pro- 

 jects freely at one period above the surface of the mucous 

 membrane, becoming subsequently included within a cap- 

 sule formed by the involution of the latter ; this occurs 

 in the human subject. The Skate offers a sort of inter- 

 mediate stage. 



The enamel forms a continuous layer, and invests the 

 crown of the tooth ; it is thickest upon the masticating 

 surface, and decreases towards the neck, where it usually 

 terminates. The external surface of the enamel appears 



Fig. 312. Tooth Structure. 



1, Longitudinal section of superior canine tooth, exhibiting general arrange- 

 ment, and contour markings, slightly magnified. 2 & 3, Portions from same, 

 highly magnified, showing the relative position of bone-cells, cementum at 2, 

 dentine fibres, and commencement of enamel at 3. 4, Dentine fibres 

 decalcified. 5, Nasmyth's membrane separated and the calcareous matter 

 dissolved out with dilute acid. 6, Cells of the pulp laying between it and the 

 ivory. 7, A transverse section of enamel, showing the sheaths of fibres, 

 contents removed, and magnified 300 diameters. 



smooth, but is always marked by delicate elevations and 

 transverse ridges, and covered by a fine membrane (Na- 

 smyth's membrane), containing calcareous matter : this 

 membrane is separable after the action of hydrochloric acid ; 

 it then appears like a network of areolar tissue, shown in 

 fig. 312, No. 6 ; which is Huxley's calcified membrana 



