56 DIGESTION AND FOOD. 



from the permanent. The latter advances, and, as shown at 

 Fig. 19, 'has a shelly aspect, seen in a. At b, the incisor 

 tooth indicates two years' wear; at c, the result of five years' 

 friction ; at d, nine years', and at /, about seventeen years' 

 wear. The shape of the wearing surface of the tooth is of 

 great importance in determining approximatively the age of 

 old horses. Before eight years of age the eruptive changes 

 and periodic appearances of the teeth are very regular and 

 valuable in indicating the horse's age. 



The foal, at birth, indicates the fast approaching eruption 

 of the two central incisors ; sometimes these are through 

 the gums when the animal is foaled; if not, they appear 

 within the first month. Three molar teeth on each side of 

 both upper and lower jaw are prominent, and in apposition for 

 wear at the same time. One incisor on each side of the two 

 central appears at six weeks, and then time is allowed for the 

 jaws to grow. The cavities of reserve with teeth forming 

 in them grow behind the teeth first formed, and by nine 

 months the corner incisors protrude, and gradually grow until 

 the animal is a twelvemonth old, when all the colt's incisors 

 are in full use. Within one and two years of age, little 

 can be seen beyond a gradual wearing down of the tempo- 

 rary teeth, and the protrusion through the gums of the molar, 

 fourth in position, on each side of the two jaws. At two 

 years the worn aspect of the incisors indicates the approach- 

 ing displacement of the central ones, and the fifth molar 

 tooth protrudes through the gums. 



Between two and three years the central permanent in- 

 cisors displace the temporary, and are readily distinguished 

 by their size, yellowish colour of the enamel, and dark infun- 

 dibulum. (Compare Figs. 10 and 12 with Figs. 17 and 18.) 

 It is at this age that the Yorkshireman often knocks 

 out the middle incisors to make the horse look "three 



