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1 5. 7^he separate uses of the three component struc- 

 tures, however, is not well shown in the incisors ; for 

 as the crusta petrosa is by a natural process removed, 

 and the ivory is not of essential service in cutting 

 the food, it might be supposed that the first was 

 no more than a temporary covering to, and the last 

 only a basement for, the enamel. When the mind, 

 however, is directed to the observation of the molars 

 or double teeth, the properties and uses of all become 

 apparent. The sense of touch in the horse resides 

 in the lips ; with those organs he gathers together 

 the food before he grasps it with the incisors. 

 Delicate, however, as the animal's sense of touch is, 

 the selection of the food is further aided by the sight 

 and smell. The most fragrant, the softest, and the 

 cleanest portions, therefore, are selected ; but it 

 would be too much to suppose, that no particle of 

 dirt, sand, or grit, was ever taken into the mouth. 

 That much is necessarily bitten, the incisors of those 

 horses which pasture on sandy soils, afford sufficient 

 proof, as such animals generally exhibit the anterior 

 edges of those teeth considerably jagged or notched. 

 In the best fields, the grass is never free from ad- 

 herent impurities, and the manger, as well as the 



