30. _The incisor teeth, in old age_, have their characteristic signs. Mostly they appear unnaturally long, though in rare instances they are worn down almost to the gums. When long, they will generally show interstices dividing them, such interstices, however, not being free, but occupied by a compact mass of foreign matter, derived from the food, and having a black colour. The reader will, by turning to the coloured plates of twenty and thirty years, see these accumulations indicated. When the incisors are very short, they are arranged in a different order to those of the young animal. The tables of the teeth of the young horse, form almost a semi-circle; in the adult animal, they gradually assume a crescentic order, and as age progresses, they ultimately range, more or less, in a straight line. The teeth also look narrower as the fangs descend with the wear of the organs, and in colour they slightly change. The enamel loses much of its semi-opaque and characteristic appearance, assuming a more dull aspect, and a yellowish tint. The crusta petrosa is to be seen only near to the gums, and within the grooves, which often are deep and very well defined, and within which the remains of the once-investing membrane becomes almost black. Then again, the tables themselves change their shape as the years increase. At first, these surfaces are of an oblong and somewhat oval figure; by degrees, however, they become angular, and ultimately more or less square. The continental authors have been very minute, and not a little profuse upon the changes of form which the tables undergo, and attribute the alteration to the gradual wear of the teeth. The correctness of this view they endeavour to establish by sawing a tooth through at various places, and attempting to show, that the forms of the surfaces exposed by this process correspond to the shapes assumed by the table during the progress of age. Their deductions can, in the study, be made to appear true, but in the stable will not bear the absolute application to which these writers would put them. By means of this test, Pessina asserted he would tell the age of a horse, accurately, up to the twenty-second year. The Girards acknowledge the tables will not guide them so far, but nevertheless are willing to apply them during the first seventeen years of the animal's life. I leave the reader to form his own opinion of the value of such speculations--for while I confess they are of some worth as guides to the primary study, and helps to the proper understanding of the cause of those changes of figure to be anticipated--I am not satisfied that any rule drawn from them can be depended upon. For that reason I shall hereafter present the reader with figures of the tables taken from authenticated mouths, rather than amuse him with speculations, which, however imposing they may seem to the ignorant, practice soon discovers to be no more than remote and plausible possibilities.