27fi THE ANGULAR OR SHARP MOUTH. 



when seen from in front, the outer edges of the lower molars are at 

 the same height as the inner edges of the upper. The grinding 

 surfaces are inclined as in the horse. Gunther states that the normal 

 rate of wear, which in herbivora is very considerable, amounts to 

 about 2 mm. (nearly one line) per year. In consequence of this 

 continued wear, the grinding and cutting surfaces should eventually 

 attain perfect contact. But where they do not, certain areas are 

 insufficiently worn, and grow abnormally fast (Exsuperantia dent is, 

 Kitt), while certain others wear too quickly. Hence arise the 

 following irregularities : — 



(a) The angular or sharp mouth. 



(b) The shear-like mouth. 



(r) The wave-formed mouth, where the row of teeth, seen 

 from the side, appears undulatory. 



(d) The step-formed mouth, where, from similar irregular 



wear, the row is composed of alternate high and low 

 teeth. 



(e) Premature wear of the teeth. 

 (/) The smooth mouth. 



THE ANGULAR OR SHARP MOUTH, THE SO-CALLED PROGNATHOUS JAW. 



Under ordinary circumstances mastication causes general wear 

 of the entire grinding surface of the molars, but if any part fails to 

 be worn, that portion will become too long. Owing to the peculiar 

 relations of the two jaws, the points which generally escape wear 

 are on the inner side of the lower row and on the outer side of the 

 upper. Either single teeth, an entire row, or even several rows, 

 may be involved. If the two rows do not terminate at the same 

 point, the upper molar being too far forward, or the first lower molar 

 too far back, the unopposed points will continue to grow until their 

 sharp elongations may injure the opposite gum. 



Causes. This irregularity may be caused by a narrow formation 

 of the lower jaw, as was shown by Defay, and later by Giinther and 

 others, though such formation is not the only cause ; otherwise 

 the ox, which very rarely exhibits the condition, should be a frequent 

 sufferer. It is assisted by limitation of the movements of masti- 

 cation, which in its turn is said by Lorge to be dependent on faulty 

 development of the muscles of the jaw. In chewing, the ox moves 

 the jaw laterally through a wide angle ; hence, although its upper 

 and lower rows of molars do not coincide, it very seldom suffers 

 from sharp edges. As soon as sharp edges form on the teeth, the 



