Common Diseases of Farm Animals

by Robert Alexander Craig

CHAPTER XIX

DETERMINING THE AGE OF ANIMALS

[Illustration: FIG. 51.—Head of young horse with bone cut away, and showing position and size of teeth.]

GENERAL DISCUSSION.—The teeth are the passive organs of digestion. They are hard organs, implanted in the superior and inferior jaws in the form of a long and narrow arch that is open posteriorly. The free portions of the teeth project into the mouth, and present sharp or roughened table surfaces for the crushing and tearing of food. In solipeds and ruminants the arch is interrupted on each side by the inter-dental space or bars (Fig. 51). The teeth that form the middle and anterior portion of the arch are termed incisors (Fig. 52). Posterior to the incisors are the canines or tusks, and forming the arms of the arch are the molar teeth. Animals have two sets of teeth, temporary and permanent. The following table gives the number of the different kinds of temporary and permanent teeth.

Temporary Teeth Permanent Teeth
Incisors Canines Molars Incisors Canines Molars

Solipeds 12 12 12 4 24
Ox 8 12 8 0 24
Sheep 8 12 8 0 24
Hog 12 12 12 4 24

The tusks or canine teeth are not always present in the female. Ruminants do not have upper incisor teeth. The temporary teeth are erupted either before or within a few days to a few months after birth. The eruption of the permanent teeth and the replacement of the temporary teeth occur at different periods up to the age of four and one-half years (Fig. 53). It is well to keep the following table of dentition in mind when examining the mouths of animals for the purpose of determining their age.[1]

Horses Cattle Hogs
Teeth Temporary Permanent Temporary Permanent Temporary Permanent

Incisors: yrs. mos. yrs. mos. mos.
Centrals At birth 2 6 At birth 1 8 At birth, 12
or 3-4
weeks
First 4-6 wks. 3 6 At birth 2 9 8-12 wks. 18
laterals
Second 5-12 days 3 6
laterals
Corners 6-9 mos. 4 6 12-18 days 4 6 At birth 9
Molars:
First At birth 2 6 At birth 2 6 7 weeks 5
Second At birth 2 6 At birth 1 6 8-28 days 14
Third At birth 3 6 At birth 3 8-28 days 13
Fourth 10-12 1 6 13
Fifth 2 2 5
Sixth 4-5 2 6 9
Seventh 18
Canines or 4-5 9
tusks

IN DETERMINING THE AGE of the different domestic animals by the development and appearance of the teeth, most of the attention is given to the lower incisor teeth. Up to the fifth year, the age of the horse or ox can be easily determined by the eruption and replacement of the incisors.

At one year of age the colt has a fully developed set of temporary incisors. The ruminant's incisors at this age all show wear.

The two-year-old colt shows a well-worn set of incisor teeth, and the ruminant at this age has replaced the nippers or centrals.

The third, fourth and fifth years are indicated by the replacement of the temporary nippers, dividers and corners in the horse, and the first and second dividers and corner teeth in ruminants.

[Illustration: FIG. 52.—Longitudinal section of incisor tooth: cup; cement; enamel; ivory; and pulp cavity.]

In the horse the permanent nippers are full grown and in wear at three years of age; the permanent dividers are full grown and in wear at four years of age; and the permanent corners are full grown and in wear at five years of age. The table surfaces of the incisor teeth of a five-year-old horse show different degrees of wear. At this period in the animal's age, the nippers have been in wear two years, the dividers one year, and the corners are beginning to show wear. In ruminants, all of the chisel-shaped table surfaces of the incisors show considerable wear when the animal is five years old.

After the animal has a full set of permanent teeth, we judge the age by the degree of wear or the appearance of the table surfaces of the incisors, their shape, the angle with which they meet and the general appearance of the head.

[Illustration: FIG. 53.—Cross-section of head of young horse showing replacement of molar tooth.]

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.]

As the table surface represents a cross section of the tooth, its appearance and shape will then depend on the portion of the tooth that it represents. From year to year, there is a gradual shortening in the lateral diameter, and an apparent increase in the diameter from before to behind. These changes in shape are from a long, narrow table surface to an oval, from oval to circular and from circular to triangular (Fig. 55). As the original free portion of the tooth wears off, the cup becomes shallow and smaller until the remnant is represented by a mere dot of enamel that finally disappears from the posterior portion of the table. After the cup has moved from the central portion of the crown and occupies a more posterior position, the dental star, which represents a cross section of the pulp cavity, puts in its appearance. It first takes the form of a brown or dark streak, and later a circular dark spot which gradually increases in size with the wear on the tooth and the age of the animal.

[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.]