EXCESS OF LENGTH OF THE CROWNS. 143 



well as longer than the lower ones, the lower jaw is obliged to 

 have a lateral movement in order to grind against the whole 

 table of the former. Wlien the horse is fed under the ordinary 

 conditions of nature, finding coarse weeds, twigs, hard vegetable 

 matters and resisting substances, as dirt, etc., mixed with its 

 food, this lateral movement is rendered complete by the slowness 

 and difficulty of mastication, and the teeth are kept worn evenly. 

 When, however, the animals are given selected fine hay, cleaned 

 oats and slops and mashes, easy of mastication, they use the 

 jaws like a chopping macliine, and all parts of the grinding 

 surfaces are not brought into contact. Again, when horses, 

 like cab-horses, doctors' hacks, etc., are fed at irregular times 

 and in a hurry, they masticate incompletely, give the jaws but 

 little lateral movement, and the narrower, lower jaws wear only 

 against the internal portion of the tables of the upper teeth. 

 When from any cause, such as caries of a tooth, cementoma, 

 disease of any kind, or deformity, the animal chews more on one 

 side than the other, the normal bevel of the teeth throws the 

 wearing surfaces from their natural position, and, while some 

 portions are worn faster, others do not receive tlie proper amount 

 of use. W^hen one or more teeth have been partially destroyed 

 or lost, the opposing teeth, finding lessened resistance, become 

 excessive in length and frequently irregular in shape. The 

 irregularity of shape from any of tlie above causes is usually in 

 the form of an increased bevel. Only the internal surface of 

 the upper molars is worn, causing the formation of a little ledge 

 along their outer borders. This acts as a check to lateral 

 motion of the lower jaw, soon prevents it altogether, and cause 

 and efiect promptly react on each other. The external borders 

 of the upper jaw become long, thin, and sharp, and their irregu- 

 lar, ragged edges cut the cheeks and serve as lodging places for 

 balls of food, which may decompose, causing further irritation 

 to the mucous membranes. The internal borders of the lower 

 teeth may become very long and stand up like so many arrow- 

 points, cutting the tongue and even penetrating the hard palate. 

 The pressure on the border of the teeth worn close to their 



