19 

 is entirely coated by this substance, having a some- 

 what dull and dark appearance, which is only lost 

 when the hard crust is removed by attrition, and the 

 enamel thereby exposed. In amount, the crusta petrosa 

 is second to the ivory, being more in quantity than 

 the enamel. In composition, it is characterized by con- 

 taining a great amount of animal substance ; and, in 

 structure, it is peculiar for exhibiting, under the micro- 

 scope, numerous corpuscles or cells, from which pores 

 or minute tubes appear to radiate. Within the alve- 

 olar cavity, the crusta petrosa, which around the fang 

 becomes of considerable thickness, is of a yellowish 

 white colour ; but where, in connexion with the crown 

 of the tooth, it is exposed to the chemical action 

 of the food and air, it presents a darker aspect 

 and looks like an accumulation of tartar, for which 

 indeed it has been mistaken. It fills up the infundibula 

 of the molars of the upper jaw, and lines those of the 

 incisors, being pierced by all the vessels which nou- 

 rish the teeth. If a tooth be subjected to the action 

 of dilute hydrochloric acid, the enamel will be remo- 

 ved, and the ivory and crusta petrosa be rendered 

 separate and distinct. 



c 2 



