As mules seldom change hands to any great extent until two or three years old, it is not deemed necessary here to say any thing of their age until they have reached two years, so as to give the inexperienced a wider scope.  The mule’s mouth undergoes exactly the same changes as the horse’s.  Between the ages of two and three these changes begin to take place in the mule’s mouth.  The front incisor teeth, two above and two below, are replaced by the horse for permanent teeth.  These teeth are larger than the others, have two grooves in the outer converse surface, and the mark is long, narrow, deep, and black.  Not having attained their full growth, they are somewhat lower than the others, the mark in the two next nippers being nearly worn out, and is also wearing away in the corner nippers.

A mule at three years old ought to have the central permanent nippers growing, the other two pairs uniting, six grinders in each jaw, above and below, the first and fifth level with the others, and the sixth protruding.  As the permanent nippers wear and continue to grow, a narrow portion of the cone-shaped tooth is exposed to the attrition; and they look as if they had been compressed.  This is not so, however; the mark of some gradually disappears as the pit is worn away.  At the age of three and a half or four years the next pair of nippers will be changed, and the mouth at that time cannot be mistaken.  The central nippers will have nearly attained their full growth, and a vacuity will be left where the second stood; or, they will begin to peep above the gum, and the corner ones will be diminished in breadth and worn down, the mark becoming small and faint.  At this period also the second pair of grinders will be shed.  At four years the central nippers will be fully developed, the sharp edges somewhat worn off, and the marks shorter, wider, and fainter.  The next pair will be up, but they will be small, with the mark deep and extending quite across.  Their corner nippers will be larger than the inside ones, yet smaller than they were, and flat, and nearly worn out.  The sixth grinder will have risen to a level with the others; and the tushes will begin to appear in the male animal.  The female seldom has them, although the germ is always present in the jaw.  At four years and a half, or between that and five, the last important change takes place in the mouth of the mule.  The corner nippers are shed, and the permanent ones begin to appear.  When the central nippers are considerably worn, and the next pair are showing marks of wear, the tush will have protruded, and will generally be a full half inch in height.  Externally it has a rounded prominence, with a groove on either side, and is evidently hollow within.  At six years old the mark on the central nippers is worn out.  There will, however, still be a difference of color in the center of the tooth.  The cement filling up the hole made by the dipping in of the enamel, will present a browner hue than the other part of the tooth.  It will be surrounded by an edge of enamel, and there will remain a little depression in the center, and also a depression around the case of the enamel.  But the deep hole in the center of the enamel, with the blackened surface it presents, and the elevated edge of the enamel, will have disappeared.  The mule may now be said to have a perfect mouth, all the teeth being produced and fully grown.