Our Humble Helpers: Familiar Talks on the Domestic Animals

by Jean-Henri Fabre

“In our talk on the Auxiliaries, I have already told you of the remarkable structure of their teeth; in particular I showed you how the horse’s molars, or grinders, are arranged so as to grind the tough fodder like mill-stones. A very hard substance called enamel, capable of striking fire like flint, covers the teeth and extends into the underlying and less resistant mass of ivory, forming on the crown of each molar a number of sinuous folds. These hard folds constitute a kind of strong file which tears in pieces the blades of forage when the opposite molar [[356]]is brought into play. Need I go over all this again?”

“No, Uncle,” replied Jules; “we all remember how ivory wears away by degrees, but the folds of enamel cap this softer substance and keep the molars in a proper condition for crushing the food.”

“Then I will continue by showing you how by examining a horse’s incisors we may learn the animal’s age. These incisors are six in number in each jaw. They are accompanied in the upper and often also in the lower jaw by two small canine teeth having the shape of pointed nipples. Beyond these, and until the row of molars begins, the jaw is toothless, and this part is called the bar.”

“I know,” broke in Louis; “it is in the bar that the bit is placed with which the horse is guided.”

“Let us return to the incisors. The two in the middle of the jaw are called the first or central incisors; the next two, one on the right and the other on the left of the first ones, are called the second incisors; finally, the two last, one on each side, are called the third incisors. Remember these names; they will save us the trouble of roundabout expressions.

“A few days after birth the central incisors show themselves in each of the foal’s jaws. In one or two months the second incisors appear, and in six or eight months the third incisors pierce the gum. These are the first or milk teeth, as they are called. When the animal is between two and a half and three years old they fall out and are replaced by the second teeth, which make their appearance in the same order as the preceding ones: first the central incisors, [[357]]then the second incisors, and lastly the third incisors. The three pairs succeed one another at intervals of about a year. I will add that the milk teeth are whiter and narrower than the others. You already see that by examining the incisors and noting whether they are first or second teeth we can tell the age of a young horse; but there are other distinctive marks which we must now learn.

Longitudinal Section of a Horse’s Incisor

“Here is a picture of the longitudinal section of a horse’s incisor. In the lower part, or root, of the tooth is a cavity occupied by the nerve which gives sensitiveness to the tooth and which carries to it, in the blood, the materials for its growth and maintenance. The upper part, or crown, likewise contains a depression, which is called the pit or cavity of the crown, and is filled with blackish matter. A layer of enamel covers the outside of the tooth, folds over the crown, and extends into the cavity, the walls of which it lines. The rest of the tooth is composed of ivory.

“From this structure you will see that the enamel, continuing uninterruptedly from the outside to the inside, forms a sharp ridge on the edges of the coronal cavity. But this condition does not last long and is found only in incisors of recent formation. In fact, by the grinding of the teeth one against another [[358]]when the animal chews its forage, the edge of the enamel first crumbles, then wears off little by little, and finally disappears altogether, leaving the ivory exposed on the top of the crown. This friction always going on, the coronal cavity or pit becomes less and less deep until at last there is nothing of it left. The upper face of the crown is then flat instead of hollowed-out as it was at first. This gradual obliteration of the hollow or pit in the crown of the incisor, whether in the first or in the second set of teeth, furnishes a means of determining the horse’s age. I have just told you when the milk teeth make their appearance; I will now add what is to be said about their wearing down. The central incisors of the first set of teeth are worn down so that their crowns are flat in ten months, the second incisors in one year, and the third incisors in from fifteen months to two years. Let us next consider how the horse’s age may be determined at a later period.