A. Thoroughbred at birth (male).

1, 2, 3. Temporary molars.

4. Permanent molars (uncut).

5, (>, 7. Crowns of temporary molars.

8, 9, 10. Central, lateral, and corner temporary incisors.

B. Four-month-old New Forest Pony (male).

1, 2, 3. Temporary molars.

4. Permanent molars.

5. Permanent molar (uncut).

6, 7. 8. Crowns of temporary molars.

9,10, 11. Central, lateral, and corner temporary incisors.

DENTITION OF THE HORSE AT VARIOUS AGES   I.

Plate LXIII. DENTITION OF THE HORSE AT VARIOUS AGES - I.

A. Thoroughbred (male) at birth. B. Four-month-old New Forest Pony (male).

No difficulty could possibly be experienced when the two organs are removed from the jaw for the purpose of examination - the difference in form and size is quite apparent; but it may also be noticed that the permanent incisor decreases in width from above downwards, without showing any line of separation between the upper part of the tooth which is called the crown, and the lower part, or fang. In the temporary incisor the distinctive boundary between the two parts is perfectly well defined. These differences are not quite so well marked when the teeth are looked at in the mouth of the living animal, but the small size of the crown, and the absence of deep grooves in the temporary incisors, will be sufficient to enable the observer to distinguish the one from the other during the period of change from temporary to permanent teeth. It is not, however, quite so easy to distinguish the temporary from the permanent incisors in ponies between one and two years of age, as these animals very frequently present some of the adult character in their general conformation; and when the judgment is to be formed entirely by an examination of the teeth, it is quite possible for one who is not an expert to mistake a well-developed pony of one year old for a five-year-old, and one of two years old for a six-year-old.

The difficulty, however, may be at once disposed of by an examination of the molar teeth.

To appreciate the variations which take place in the outline of the worn surfaces or tables of the incisor teeth as age advances, it is desirable that the examiner should have a clear understanding of the peculiarities of form in these organs, which render a certain fixed series of changes in the outline of the tables quite inevitable. In the next figure (fig. 601), representing a permanent incisor of a horse, it will be seen at once that the long diameter of the table is exactly at right angles with the long-diameter at the base of the tooth. Further, it will be seen in the drawing, which shows the tooth as seen from the back, that a ridge extends from near the upper surface to the bottom of the tooth, causing the opening at the base of the fang to represent a triangular figure. Consequently, sections commencing at the upper part of the tooth, and carried down to the bottom of it, would represent the forms which are shown on the right-hand side of the drawing. The wearing of the teeth from the upper surface downwards is compensated by the growth of the tooth upwards from its cavity, and the changes in the form of the table as the animal advances in life are necessarily a gradual approach to the triangular form.

Permanent Incisor, showing sections at various points.

Fig. 601. - Permanent Incisor, showing sections at various points, a, The central cavity.

Besides the changes in the form of the table which the tooth undergoes during the course of its natural wear from constant attrition, there are also changes relating to the "mark", which is the name in common use to indicate the cavity in the centre of the tooth, which becomes dark or black in colour from the action of the food on the bony structure. The cavity is formed by the inversion of the three structures of the tooth, the crusta, enamel, and ivory.

The hollow cone extends about half-way down the incisor tooth, and consequently, when the wear reaches to a certain point, the cavity is obliterated, or, in the horseman's language, the mark is worn out.

One result of the inversion of the tooth structures to form a hollow cone in the interior of the incisor is a peculiar arrangement of lines on the worn surface. This condition is shown in fig. 601. An outer line of white enamel is seen, inside which is the broader line formed by the bulk of the tooth - the ivory; then the inner line of enamel which belongs to the inverted cone, with the lining of crusta, which is originally on the outside of the tooth, but in the inverted structure is necessarily on the inside. This structure is quickly darkened by contact with food. The darkening is also distinctly seen on the crusta on the outside of the tooth, being especially marked in the grooves. The prominent parts of the surface become white in consequence of the darkened crusta being rubbed off by the movement of the animal's lips. It will be obvious that the table of the tooth exhibits the following features:-

1. An outer ring of enamel. The outer covering of crusta is at the edge of the tooth worn away.

2. A broad line of ivory, in the centre of which is a faint line, showing the junction of the inverted cone with the outer shell of the tooth.

3. A ring of enamel called central enamel, with the line of dark crusta inside it.

It is necessary to note here that the changes in form of the central enamel afford important evidence of the age after seven years.