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Show 434 MR. J. B. SUTTON ON HYPERTROPHY. [May 5. functional activity. The process by which this very desirable result has been attained is distinguished by the convenient term hypertrophy. Having illustrated what I understand by that term, let me now define it as " the enlargement of an organ beyond its usual limits as the result of increased function, or of some unusual condition of the corresponding or correlated organ." The majority of instances of hypertrophy, usually recognized as such, are the result of disease. In the example just considered, the enlargement of the persistent kidney is the direct outcome of atrophy of its fellow ; but if a careful inquiry be instituted, many conditions, which at first sight appear to be the result of perpetuated hypertrophy, in consequence of atrophy of neighbouring structures, will really admit another interpretation, viz. that the hypertrophy is in great measure responsible for the atrophy of those structures, and bears a definite relation to it. No better example offers itself whereby m y meaning may be elucidated than the middle metacarpal of the Horse. The metacarpus of the Horse is composed of a greatly hyper-trophied third metacarpal bone, whilst applied to the posterior lateral edges of this large bone may be seen two rudimentary metacarpals. Each of these bones represents a slender pyramid with its base upwards and the apex pointing directly downwards. Of the two bones the internal is thicker than the external, and often the longer of the two. The large bone represents the middle or third metacarpal, whilst its two slender companions correspond to the second and fourth metacarpals of the typical mammalian manus. Thanks to the labours of Rutimeyer and Marsh, we are now in possession of the palseontological history of the ancestors of our modern Horse. The facts on this point, so far as regards the manus, have been admirably summed up by Wiedersheim ' as follows :- The oldest known stem form of the Horse is Eohippus, which has been found in the Lower Eocene of North America. It possessed four well-developed fingers and the rudiment of a fifth, the thumb. Next we find that the thumb has completely disappeared in Oro-hippus and Epihippus, the four remaining fingers peisisting. These animals were about the size of a Fox, whilst in the Upper Eocene Mesohippus had attained the size of a Sheep. Here three fingers only were well formed ; a fourth merely existed as a rudiment, which in the next form, Miohippus, has become yet smaller. In the Pliocene representative, Protohippus, the last remnant of a fourth finger disappears and three only persist. This animal corresponds to the European Hipparion, and was of the size of an Ass. Another Pliocene form, Pliohippus, had the second and fourth fingers extremely rudimentary, the development of the third being alone significant. Lastly, we come to the modern Horse, with its peculiarly specialized manus. The accompanying figures are intended to afford a graphic representation of the preceding facts (fig. 2, p. 435). 1 Vergl. Anatomie, vol. i. p. 198. |