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 hyper- 

 tropliy. 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 condi- 

 tion of the corresj)onding or correlated organ." 



The majority of instances of hypertrophy, usually recognized as 

 such, are the result of disease. In the example just considered, tlie 

 enlargetnent of the ])ersistent kidney is the direct outcome of atropliy 

 of its fellow; but if a careful inquiry be instituted, many conditions, 

 which at first sight appear to be the result of perpetuated hyj)er- 

 troph)', in consequence of atrophy of neighbouring structures, will 

 really admit another interpretation, viz. that the hyj)ertrophy is in 

 great measure responsible for the atrophy of those structures, and 

 bears a definite relation to it. 



No better example offers itself whereby my meaning may be eluci- 

 dated than the middle metacarpal of the Horse. 



The metacarpus of the Horse is comjjosed of a greatly hyper- 

 trophied third metacarpal bone, whilst applied to the posterior 

 lateral edges of this large bone may be seen two rudimentary meta- 

 carpals. 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 tiie external, and often the 

 longer of the two. Tiie large bone rej)resents the middle or third 

 metacarpal, whilst its two slender companions correspond to the 

 second and fourth metacarpals of the tyjjical mammalian manus. 



Thanks to the labours of Riitimeyer 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 fol- 

 lows : — 



The oldest known stem form of the Horse is Eohippus, which 

 hasl)een 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 tliat the thumb has completely disappeared in Oro- 

 hippus and Epihippits, 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, Frotohippus, the last remnant of a 

 fourth finger disappears and three only persist. This animal cor- 

 responds to the European Hippario7i, and was of the size of an Ass. 

 Another Pliocene form, Fliohippus, had the second and fourth 

 fingers extremely rudimentary, the develojiment of the third being 

 alone significant. Lastly, we come to the jnodern Horse, with its 

 peculiarly specialized manus. 



The accompanying figures are intended to afford a graphic repre- 

 sentation of the preceding facts (fig. 2, p. 43.t). 

 ' Vergl. Anatomie, vol. i. p. 198. 



