136 AGE OF THE DOMESTIC ANIMALS. 



shortening each year, from the wearing away of its table ; this 

 even length is maintained by the roots being constantly forced 

 from the alveolar cavities and becoming a part of the free 

 portion; transverse lines, filed at definite points on the an- 

 terior surface of the incisors, will be seen to approach the 

 table and disappear, one after another. Experiments by Pessina 

 showed that in common horses the annual wearing is about 

 4J millimetres, while in thoroughbreds it is only about 3 milli- 

 metres ; experiments by Bouley verified those of Pessina. 



Girard, after numerous investigations, determined that the 

 free portions of the incisors, from the gum to the table, average 

 15 millimetres. This is subject to some variation, but normal 

 pinchers rarely vary from 18 millimetres in length, intermediate 

 teeth beyond 15 millimetres, while corner teeth are about 13 

 millimetres. Variation in the length of the incisors may be from 

 excess of length or from deficiency of length. 



EXCESS OF LENGTH. 



This anomaly occurs in several ways, either occurring in 

 both jaws, or in the teeth of the upper jaw alone, or at times 

 only in certain teeth of one or the other jaw. 



EXCESS OF LENGTH OF BOTH JAWS. 



When the teeth of both jaws are too long, there is a general 

 . tendency for them to become parallel ; that is, to approach a hori- 

 zontal direction ; but their free extremities are diverged like the 

 ribs of a fan, and the wearing surface, or table, shows the char- 

 acter of the age of the horse ; flattened in front to behind, they 

 do not tend to take an oval form ; the central enamel occupies a 

 large portion of the dental table. There is often a little external 

 cavity in the inferior corner teeth ; the excess of length of the 

 crown of the tooth is not in proportion to that of the root ; the 

 teeth are less sohdly fixed in their alveolar cavities, and are sub- 

 ject to fracture from moderate violence, on account of the great 

 leverage of the lengthened crown. These teeth on their table 

 apparently show an age which is sometimes very deceptive. 



