1, External border; 2, internal border; 3, semilunar crest; 4, 4, re-entrant processes.] As an indispensable complement to the study which we have just made, it is necessary to add that the hoofs of the fore-limbs and those of the hind ones present differences of form which cannot be ignored--differences which we are already able to conjecture by looking at the respective third phalanges which terminate those limbs, and especially at their inferior surfaces (Figs. 101, 102). The hoofs of the fore-limbs (see Fig. 100), viewed on their plantar surface, are more rounded than those of the hind-limbs (Fig. 103)--so that their external contour may be compared to a semicircle--whilst the hind-hoofs, which are narrow and of more oval shape, rather recall by their form the aspect of an ogive. This seems to result from the fact that the fore-limbs support the more considerable part of the weight of the animal. The best proof which can be given of this overweighting is the eagerness with which very often, when a horse is stopped near the edge of a footpath, for example, he places his fore-feet on the latter. In thus raising his fore-quarters, he throws part of his weight backwards, and in this way relieves his fore-limbs. [Illustration: FIG. 103.--INFERIOR SURFACE OF A HIND-HOOF OF A HORSE: LEFT SIDE. 1, External border; 2, internal border.] With regard to the difference of form which we have just pointed out, we have sometimes heard the following comparison made: the contour of the hoofs of the fore-limbs, viewed from below, recalls that of an apple; that of the hoofs of the hind-limbs recalls the outline of a pear. As a mnemonic this comparison is insufficient, for nothing connects either of the forms indicated with the region to which the hoofs belong. We much prefer one made for us this very year by one of the students of our course at the School of Fine Arts, after the lecture in which we had just pointed out the differences in question. Giving the idea of a semicircle and an ogive, which we described above, he remarked to us that the idea would perhaps be more easily fixed in the memory if we associated with it the idea of the chronological order in which the Roman and ogival art succeeded. Indeed, as the Roman art preceded the ogival art, so the hoofs which have the semicircular form precede those which have the form of an ogive. This interpretation appeared to us ingenious; this is why we wished to give it here a place which seems to us to be merited. [Illustration: FIG. 104.--LEFT POSTERIOR FOOT OF A HORSE: EXTERNAL ASPECT.] The wall of the hoof of a fore-limb, viewed on one of its lateral surfaces (see Fig. 98), is more oblique than that of one of the hind-hoofs looked at in the same way (Fig. 104). This difference, very marked especially at the region of the toe, is correlated with that of the direction of the pastern. In fact, in the anterior limbs this is a little more oblique than in the opposite ones.