When the dentition is completed at the age of five years, the horse has six incisors or nippers in the front of the mouth, in the top and bottom jaws, and six molars on each side, top and bottom jaws. The three last of the row are true molars, the three in front of them are distinguished as pre-molars. In addition, in front of the anterior pre-molars on each side of the top jaw there is often seen a small conical tooth, which, notwithstanding its insignificant appearance, is in the popular view an organ of some importance. The term eye-teeth is generally applied to these rudimentary organs, and it is believed, even by people who ought to know better, that the presence of this tooth in some extraordinary way is a cause of blindness, and in the case of a horse of three or four or five years of age having any disease in the eyes, it is usual to look in the animal's mouth in order to see if the eye-teeth, or, as they are sometimes called, "wolves' teeth ", are present. If so, they are immediately removed by a somewhat primitive method of punching. An ordinary punch, which is used for the preparation of nail holes for the horse's shoes, and the shoeing hammer are found to be effective instruments for the operation. As these rudimentary organs have a very slight hold in the jaw, a very moderate amount of force will dislodge them, and the horse is neither better nor worse for the performance.
Reference to the section on the conformation of the horse will convince the reader that the small conical tooth, to which so much importance is attached, is really the vestigial remains of the first pre-molar, which is a well-developed tooth in the top and bottom of both sides of the mouth in many of the ancient ungulate mammals, making a row of seven instead of six molars, of which four were pre-molars and three true molars. The gradual diminution in size of the first pre-molar may be traced in the fossil remains of horse-like animals of the tertiary formation. In the horse of the present time the first pre-molar has altogether ceased to exist in the bottom jaw, and only remains in the top jaw as a rudimentary and occasional structure, which is frequently shed when the temporary pre-molars are exchanged for permanent. A peculiarity in the horse's mouth more difficult to account for than that above referred to is the space which exists between the molar teeth and the incisors. This space did not exist in the most ancient mammals, but in the Phena-codus there were some indications of it, and it becomes more distinct through the series of horse-like animals which will be described in the chapter on the peculiar features of the conformation of the horse. In the male of the horse family the space is partly occupied by the canine teeth or tusks; in the mare these organs are either entirely absent or are merely rudimentary. Form and Arrangement. - Some knowledge of the form and general arrangement of the different orders of teeth are essential for an intelligent appreciation of the changes which take place owing to wear in one direction, and the growth of the organs in the other.
The incisor teeth are chiefly used as a means of judging the age after permanent dentition is complete. Up to that time the change from temporary to permanent organs, both incisors and molars, affords important indications of the age of the animal from birth up to the age of five years.
For the purpose of distinguishing the temporary from the permanent organs, an illustration will be more useful than a written description, and in fig. 600 the temporary and permanent incisors of the horse are shown side by side.
Fig. 600. - Permanent and Temporary Incisors of Horse.