HISTOLOGICAL DEMONSTRATIONS. 89 



wavy lines running from within outwards in the dentine are 

 the dentinal tubules. The arched incremental lines which 

 may sometimes be seen crossing the dentinal tubules in the 

 crown of the tooth indicate a lamination of the dentine 

 apparently due to incomplete calcification of its matrix 

 during development. The enamel consists of prisms set 

 with their ends on the dentine. Lines more or less con- 

 centric, and of a brown colour, may often be seen crossing 

 the enamel prisms. Their cause is uncertain, but they look 

 as if they resulted from the deposit of a brown pigment. 



(H.) The enamel prisms are arranged in bundles that 

 often decussate at their inner ends near the dentine. In 

 the preparation there may be some of these bundles cut 

 transversely. These show the fibres to be more or less 

 hexagonal. The dentinal tubules have primary dichotomous 

 divisions, and give off great numbers of fine lateral branches. 

 The latter are usually very evident in the outer part of the 

 dentine of the fang of the tooth. In the crown, very pro- 

 bably, minute rounded apertures in the transverse sections 

 of the dentinal tubules may be found. These are, how- 

 ever, most easily seen in a V. S. of dentine made at one 

 side of the pulp cavity. Interglobular spaces may perhaps 

 be seen in the outer part of the dentine. These are irre- 

 gular cavities, which, like the incremental lines that fre- 

 quently open into them, apparently result from defective 

 calcification. In such a preparation as this they have a 

 black appearance, resulting from their being filled with 

 de'bris during the grinding of the tooth. The crusta petrosa 

 is merely bone. 



132. Structure of Softened Tooth. V. S. jaw of 

 cat softened in dilute chromic and nitric acid, as directed 

 in 15. Mount in Farrants' solution or in glycerine jelly. 

 The latter is preferable. 



(L.) The tooth in the alveolus. The periodontal mem- 

 brane serving as a periosteum for the crusta petrosa and 

 the interior of the alveolus. If the section be one of the 

 lower jaw, the canal containing the inferior dental nerve 

 and blood-vessels will be cut across, and good sections of 

 these structures thus obtained. All the parts of the tooth 



