TEETH. 79 
three components; bone or ivory constitutes the chief bulk of the organ, 
and over that is spread a thin covering of inorganic enamel, the whole 
being invested with a fibrous coating of crusta petrosa. The enamel is 
the material on which the tooth depends for its cutting properties; the 
manner in which the edge is preserved deserves attention, for the brick- 
layer’s trowel appears to have been suggested by it. A thin coat of 
hard but brittle enamel is held between the two other bulky and tough 
substances, just as a thin layer of steel is protected by coatings of yield- 
ing iron in the house-builder’s instrument. 
The highly organized crusta petrosa is often injured; to understand 
this, we must first comprehend the vast power which urges the jaw of 
the horse. The motion resides entirely in the lower portion of the skull, 
which is moved by strong, very strong muscles, going direct from their 
attachments to their insertions. No force is lost by the arrangement, and 
no less a motor power was required to comminute the hays and oats on 
which the horse subsists. The machinery seems to be admirably adapted 
to its purposes; and to be so strongly 
framed as to defy all chance of injury. 
Man, however, has a mighty talent for evil; 
it does not always suit the convenience of 
the groom to sift the pebbles from the grain ; 
corn and stones are hastily cast into the man- 
ger, and the poor horse, having no hands to 
select with, must masticate all alike. The 
reader can imagine the wrench which will 
ensue, when a flint suddenly checks the aS HR SER Wino REC 
movement of the molar teeth. The crusta 
petrosa is bruised upon the large fang of the tooth. Disease is estab- 
lished, and sad toothache has soon to be endured. 
Then there are the effects of the 
powerful acids in much favor with 
most grooms and too many veteri- 
nary surgeons; moreover, there are 
the sulphates, which in every pos- 
sible form enter into veterinary 
medicine; the nitrates, likewise, are 
much esteemed, and are given in 
enormous doses. All of these much A HORSE QUIDDING, OR ALLOWING THE FOOD 
affect the crystalline enamel of the MASTICATION. ea ert one 
molar tooth; a small hole is first 
formed; into this the food enters and there putrifies; caries and tooth- 
ache are the result. 
