62 STABLE MANUAL AND HOHSE DOCTOR 



The teeth are built up of three distinct substances, 

 distinguishable from each other by the naked eye, and 

 differing materially in density, hardness, and composition. 

 They are known as the ivory, the enamel, and the crusta 

 petrosa (or stony crust). 



The ivory (called hony or horny substance, by old 

 writers) constitutes the larger portion of the bulk of each 

 tooth, and has numerous small pores or cavities. Like the 

 small canals in the substance of bones, they seem to contain 

 some colourless fluid, which nourishes or maintains the part 

 in which they are situated. The ivory is by no means so 

 close and hard as the noticed substance, the enamel. 



The second substance, the enamel, is so close, hard, and 

 homogeneous as to seem without animal matter. It is 

 thin, white, and somewhat transparent, and so hard that it 

 will strike fire with steel, like flint. 



The third substance is the crusta "petrosa^ or stony coat. 

 This forms the outer covering of the tooth, and having 

 a dirty yellowish or dark appearance, was mistaken by 

 ignorant farriers for a deposit of tartar, or " fur." It 

 contains a great proportion of animal substance, and, 

 under the microscope, is full of small vessels or tubes. It 

 is plentiful in the aveolar cavities, and is there yellowish- 

 white ; but when at the neck of the tooth, above the gum, 

 it becomes exposed to the chemical action of the air, the 

 animal juices, and the food, it receives a dark stain, and 

 looks like an accumulation of tartar. The crusta petrosa 

 will be found filling up the pits (infundibula) of the grind- 

 ing teeth of the upper jaw, and lining the top cavity of the 

 incisors. It is full of vessels for nutriment and increase. 

 Let us now consider the use of these three structures of the 

 teeth, of various densities, cutting capabilities, and power 

 to resist friction. 



The outer coat, or crusta petrosa, which at first covers 

 the tooth, is soon rubbed away from the greater part of the 

 sides, so as to show the enamel. It is not reproduced 

 there, but remains round the neck of the tooth, and appears 

 to enact a part within the aveolar cavity, in which the 

 fang or base of the tooth is situate. It is with the ivory 

 and enamel, however, which build up the substance of the 

 tooth, we have most to do. 



Upon the ivory, and its wear in relation to the enamel, 

 depends the great criterion, the presence, in a greater or 



