Parrot-mouth, Lantern-jaw and Scissor-mouth.—The common deformities of the jaw and teeth are the overshot or parrot-mouth, the undershot or lantern-jaw, and the scissor-mouth. These different deformities result in unequal wear on the table surfaces of the incisors and molars. In both the overshot and undershot jaws, the incisor teeth become abnormally long. In the parrot-mouth, the wear occurs on the posterior face of the superior and the anterior face of the inferior incisors, the teeth becoming worn to rather a sharp edge, depending on the degree of the deformity. In the lantern-jaw, the wear occurs on the posterior face of the lower and the anterior face of the superior row of incisors, the teeth taking on somewhat the same shape as the parrot-mouth. The greater the deformity and the older the horse becomes, the more difficult it is for the animal to feed or graze on pasture.

In all horses, the two rows of molar teeth are wider apart in the superior than in the inferior jaw. This results in the external border of the tables of the superior row of molars becoming longer, or projecting further downward than the internal border. The wear on the table surfaces of the inferior row of molars is just the opposite of the superior row. In the scissor-mouth the wear takes place largely on the internal face of the superior and the external face of the inferior row of molars. The teeth become worn to more or less of a blunt cutting edge, and after a time the molars come together somewhat like the jaws of a pair of scissors. A horse with a badly deformed scissor-mouth is unable to grind the feed, and unless given special care, suffers severely from innutrition.

The treatment of deformed mouths consists in removing the irregular or unworn portion of the teeth by means of the tooth float and cutters. This attention should be given early before the free portion of the tooth has become excessively long and irregular. This should be followed by dressing the teeth every six or twelve months.

SHARP LATERAL BORDERS ON THE MOLAR TEETH.—This is a very common condition in horses. The external border of the superior and the internal border of the inferior row of molars wear away slowly, and sometimes become quite sharp. This is objectionable because the sharp points lacerate the mucous membrane of the cheek and tongue, and the mastication of the feed is seriously interfered with.

This condition is caused by an excessive difference in the width of the jaws, unusually prominent ridges of enamel on the external face of the superior molars, and any conditions that may limit the movements of the jaw.

The following symptoms may be noted. The animal has difficulty in masticating the feed because of injury to the cheeks or tongue by the sharp points of enamel. This condition may be indicated by holding the head to one side. Salivation is usually present. Acute indigestion and innutrition may occur.

By examining the teeth, their condition can be determined. The sharp borders may be removed by dressing or floating the teeth. It is advisable in the majority of horses to float the teeth at least once in every twelve months.

IRREGULARITIES IN THE TABLE SURFACE OF THE MOLAR TEETH.—Horses eight years of age or older frequently have irregular molars (Fig. 56). This is due very largely to the difference in the quality of the teeth. The harder molars do not wear off as rapidly as the softer ones. This results in the table surfaces of the rows of molars becoming wavy or step-like in outline. Sometimes the first or sixth molar overhangs or projects beyond the corresponding tooth of the opposite jaw. When this occurs, the over-hanging portion may become long and sharp. A molar tooth becomes excessively long if the opposite one is decayed or removed.

The symptoms are very much the same as when the borders of the molars are sharp.

[Illustration: FIG. 56.—Teeth showing uneven wear occurring in old horses.]

The treatment consists in levelling the tables as frequently as necessary by cutting off the longer projections, and removing the sharp edges with a tooth float.

SMOOTH MOUTH.—In old age the tables of the molar teeth may become so smooth that the horse cannot grind or masticate the feed. When all of the molars are in this condition, a rubbing sound may be noted when the animal is masticating hay. After attempting to chew the hay, it may be dropped from the mouth. Innutrition always occurs.

The treatment consists in feeding chops and soft feeds.

DENTAL DISEASES.—Inflammation of the alveolar periosteum is a common dental disease in domestic animals. This is an inflammation of the alveolar dental membrane that fixes the tooth in the tooth cavity.

Injuries to the gums and cracks or fissures in the tooth are the common causes. Caries or tooth decay is not uncommon. The predisposing factor is a poor quality of enamel and dentine. The process of decay is assisted by microorganisms.

The early symptoms may escape notice. Slobbering, masticating on one side, holding the head to one side, retained masses of food in the mouth and a disagreeable odor frequently occur. Caries may be indicated at first by a dark spot on the table of the tooth, later by a cavity. In horses, inflammation of the alveolar membrane results in a bony enlargement on the side of the face if the superior molar is involved. A swelling of the jaw and fistula may result if a lower molar is involved (Fig. 57).

[Illustration: FIG. 57.—Fistula of jaw. This condition was the result of neglected treatment of decayed teeth.]

The treatment consists in the prompt removal of the tooth. This is more difficult in young animals than it is in the middle-aged or old. Unless the tooth is already loosened it may be necessary to remove it by trephining.

QUESTIONS

1. Describe the appearance of the teeth in an overshot or undershot jaw.

2. Describe the appearance of sharp molar teeth; a scissor-mouth.

3. What are the causes of decayed teeth?