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Show 1885.] MR. J. B. SUTTON ON HYPERTROPHY. 435 With such an array of evidence no one would venture to deny that the slender fourth and second metacarpals are rudimentary structures in the true sense and meaning of that term. These bones are often a nuisance to the animal, as they are exceedingly liable to inflame and produce lameness ; the disease is known to veterinarians as splent-bone, and often ends in osseous ankylosis between them and the main metacarpal, a condition of things detrimental to the Horse for working purposes. The history of this foot has been given, for it serves to illustrate a principle-hypertrophy of one structure leading to the atrophy of another; for in this case it seems the most probable view, that gradual increase in the size and functional importance of the third metacarpal bone has led to the abortion of the remaining bones, its original companions, by causing a diversion of the nutrient stream to its own advantage, but to the detriment of the lateral metacarpals. Fig. 2. A series of figures to show the ancestry of the manus of the modern Horse. I. Orohippus (Eocene). 2. Mesokippus (Upper Eocene). 3. Miohip-pus (Miocene). 4. Protohippus (Upper Pliocene). 5. Pliohippus. 6. Equus (after Wiedersheim). In no structure is this principle so admirably illustrated or so easily studied as in the teeth of mammals. Take, for example, the curiously modified teeth of the Walrus ; we shall find the enlarged and formidable incisors have for their companions some tiny func-tionless teeth of no use whatever to the animal: as a rule they quickly fall ; even when they persist, there are scarcely any sockets provided for their reception. In this case it can hardly be denied that hypertrophy of the canines has led to atrophy of the remaining teeth. The Felidae supply us with similar examples. The large size to which the canines attain in this group leads to atrophy of the teeth immediately adjacent to them. This process seems, so far as the Felidte are concerned, to have reached its culminating point in the extinct Macharodus, or Sabre-toothed Tiger, in which the canines have attained considerable dimensions, whilst some of the premolars in the upper and lower jaws have disappeared. Let us now consider some other examples of hypertrophied teeth somewhat different from the last. The extraordinary canines of the male Babirusa have afforded plenty of scope to those imaginative |