There are several different factors that may cause the wear on the teeth, and the appearance of their table surfaces to vary in the different individuals. The two factors that are of the most importance are the quality of the teeth and the character of feed. Soft teeth wear more quickly than hard teeth, and the teeth of horses that feed over closely cropped and sandy pastures wear rapidly because of the dirt and grit present on the short grass. This variation in the wear is of little importance to the person who must judge the age of a horse that he expects to purchase by the condition of the teeth. In reality, a horse is just as old as the wear on the teeth and his general appearance indicate. In order to stand severe work the animal must be able to masticate the feed, and prepare it for digestion in the stomach and intestines. The degree of wear on the molar teeth may be indicated by the wear on the incisors. The general condition of the horse and his ability to stand hard work depend very largely on the condition of the table surfaces of the molars. It is very difficult to judge the age of horses that have deformed mouths or that are in the habit of crib-biting, because of the irregularity in the wear of the incisors. When examining the teeth for the purpose of determining the horse's age, the shape of the incisors, the angle with which they meet and the appearance of their table surfaces should be observed. The teeth of young horses show more or less yellowish cement. At about seven years of age the anterior faces of the teeth are usually white, later a yellowish color. The teeth of middle-aged horses may be long, and in aged animals, narrow and short. The incisors meet at a more acute angle in old than young horses. _The free portion of the incisor_ tooth is flattened from before to behind. At the level of the gums its two diameters are about the same, but the portion of the tooth imbedded in the jaw bone is flattened from side to side. As the tooth becomes worn off, the length of the free portion is maintained by a pushing out of the tooth, and a corresponding shortening of the portion that is fixed or imbedded in the jaw. _The table surface of the unworn incisor_ tooth is covered with enamel, and in the middle portion the enamel forms a deep cup. After the tooth has become worn the margin of the table portion is then limited by a ring of enamel. This is termed the encircling enamel ring. The central portion of the table shows a second ring, the central enamel ring, that limits the cup margin (Fig. 54). [Illustration: FIG. 54.--Transverse section of incisor tooth: peripheral cement; peripheral enamel; ivory; central enamel; and central cement.]