134 AGE OF THE DOMESTIC ANIMALS. 



IRREGULARITIES FROM FAULT OF LENGTH OR EXCESS OF SIZE 

 OF ONE OF THE JAWS. 



Deficiency in length of either of the jaws is rare in the 

 horse, but sometimes occurs. The simplest form is where there 

 is a slight projection of the lower jaw (prognathism in man) ; 

 in this there is excessive wearing of the lower incisors, which 

 complicates the determination of age (Fig. 143). A more seri- 

 ous abnormality is deficiency in length in the lower jaw (brachyg- 

 natliism in man), which is followed by deformity of the inter- 

 maxillary bones ; these curve down and interfere greatly with 

 the prehension of food, especially ^t pasture. 



In prognathism the lower table may project for a variable 

 distance beyond the upper jaw, but it is rare that it extends 

 beyond to any marked degree, and, while it causes the teeth to 

 wear away rapidly, there is not often any alteration in the inci- 

 dence of the jaws. In brachygnathism the difference in length 

 of the two jaws may be very great, in some cases so great that 

 the upper jaw completely overreaches the lower one, and by 

 bending of the intermaxillary bones the upper teeth drop over 

 the anterior face of the lower teeth, while the latter point their 

 tables directly into the hard palate ; in other cases the posterior 

 face of the upper incisors wears against the anterior face of the 

 lower incisors until both are beveled into sharp wedges (Fig. 144). 



EXCESS OF WIDTH OF THE UPPER INCISIVE ARCH. 



This condition is normal in old horses, without incon- 

 veniencing them. It sometimes occurs in young horses, in which 

 the irregular wearing against the upper teeth produces marked 

 notching and sometimes interferes with the prehension of food ; 

 in one case, in a well-bred Hambletonian mare, in which the 

 excess of breadth of the upper incisive arch was due to the 

 presence of supernumerary corner teeth, giving the arch eight 

 teeth in all, the deformity was very marked. 



IRREGULARITIES BY EXCESS OR FAULT OF USE. 



As a general thing the incisive teeth have the same length 

 in their free portion, although the whole tooth is constantly 



