[Illustration: FIG. 55.--Table surfaces of nippers at different ages: four years; six years; nine years, and fifteen years of age.]
The following changes in the shape and appearance of the incisor teeth of the average horse occur in the different years. Unless otherwise mentioned, the statements made regarding the appearance and wear on the table surfaces apply to the lower incisor teeth.
SIX YEARS.--The table surfaces form the most accurate guide. The cups of the nippers tend to an oval form. The corner teeth have been in wear one year at this time. The cup is deep and the posterior margin may show little wear. It is not uncommon to meet with corners that possess irregularly developed tables, and have cups with posterior margins that are thin and do not come into wear until later. For this reason, it is not best to depend on the appearance of the corner teeth alone.
SEVEN YEARS.--The teeth are usually whiter than the previous year. The profile of the upper corner teeth shows a notch in the posterior portion of the table surface. This is due to the superior corners overhanging the inferior corner teeth posteriorly, resulting in this portion not wearing away. This notch is sometimes slightly in evidence the previous year. The cups in the corners are smaller and the worn surface larger than at six.
The nippers show oval table surfaces and the dividers are beginning to take on this shape. The shifting of the cups toward the posterior portion of the tables of the nippers and dividers is noticeable.
EIGHT YEARS.--As viewed from the side, the profile of the teeth shows a very noticeable increase in the obliquity with which they meet. The posterior borders of the corners show considerable wear. The notch in the superior corners is still present, but as the teeth come more nearly in apposition it may begin to disappear. All of the inferior tables are level.
The nippers and dividers are oval in shape, and the cups have become decidedly narrow. The nippers show a well-defined dark streak just in front of the cups. This is the beginning of the dental star.
NINE YEARS.--The appearance of the table surface is more characteristic at this time than the previous year. The cups are less prominent and the plainness or smoothness of the inferior table is more noticeable. The nippers are round, the cups triangular and the dark streak narrower and more distinct than the previous year. The dividers are becoming round and the corner teeth are oval.
TEN YEARS.--The teeth are more oblique than in the eight-year-old and nine-year-old mouth. The table surfaces of the inferior nippers are decidedly rounded, the cups are small, triangular and situated well toward the posterior borders. The dark brown streak or dental star is situated in the central portion of the nippers and dividers. The tables of the dividers are round.
ELEVEN YEARS.--The tables of the corner teeth are rounded. The dark streak or dental star is present in all of the teeth, and the remnants of the cups appear as small rings or spots of enamel near to the posterior borders of the tables. The notch in the superior corners may reappear at this time.
TWELVE YEARS.--The profile of the teeth when viewed from the side is quite oblique. The table surfaces of all the incisors are round. But a trace of the cup remains in the inferior incisors. The head of the animal is beginning to show age. The inferior border of the jaw bone appears narrower, or sharper than in the young horse.
THIRTEEN YEARS.--All of the specks of enamel or the remnants of the cups are gone from the lower incisors. A larger notch may be present in the upper corner teeth than at twelve. The tables of the inferior nippers are becoming triangular and show a small, dark spot or dental star.
FOURTEEN YEARS.--The tables of the inferior nippers are triangular, and the dental star appears as a dark round spot in both the nippers and dividers.
FIFTEEN YEARS.--The angle with which the teeth meet is greater than at twelve, the teeth are smaller and dental stars are represented by dark round spots in all of the inferior incisors. The tables of the nippers and dividers are triangular.
SEVENTEEN YEARS.--All of the tables of the lower incisor teeth are triangular. The teeth are narrower and smaller than at fifteen. The profile of the incisors, viewed from the side, is quite angular. The dental stars are prominent.
NINETEEN YEARS.--All of the signs of the seventeen-year-old mouth are more prominent. The cups have usually disappeared from the upper incisors.
[Footnote 1: This table is from dentition tables given in "Age of the Domestic Animals," by Huidekoper.]
QUESTIONS
1. Name the different kinds of teeth; state the arrangement and number.
2. How is the age of an animal determined?
3. Give the time of replacement of the temporary incisor teeth.
4. How is the age of the animal determined between the fifth and ninth years?
5. What changes in the appearance of the table surfaces occur between ten and fifteen years of age?
CHAPTER XX
IRREGULARITIES OF THE TEETH
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?