41 



not even been developed. The horse is seven years 

 old before all the roots are perfected, and when these 

 are completed, I doubt if the after growth is mate- 

 rial. If the jaw of an old horse be examined, the 

 alveolar cavities will be found to be shallow, showing 

 that the loss, consequent upon the wear, was com- 

 pensated by the tooth being projected into the mouth, 

 and not by any increase of substance. Again, those 

 animals, the incisors of which retain the marks in 

 extreme old age, may show long teeth, but not of 

 that excessive length which growth would suppose ; 

 only such as the want of wear would occasion, sup- 

 posing the increase to cease when the root was per- 

 fected. The fang, in fact, is so much tooth in 

 reserve, and as such answers the purpose for which 

 growth was supposed to be necessitated. I have by 

 me, specimens of old teeth, but the measurement of 

 none of them contradicts the opinion I have ad- 

 vanced, there being but a material increase of the 

 crusta petrosa, which at the root blends with the 

 ivory, and cannot be clearly separated from it. The 

 thickening of the crusta petrosa around the root and 

 neck, probably never ceases during the health of 

 the animal ; but to this substance alone is confined 



