180 TITE INDUCTIONS OF BIOLOGY. 



hands, find that such an exercise as rowing, soon begins to 

 produce a like thickening. This relation of cause and effect 

 is still better shown by the marked indurations at the ends of 

 a violinist's fingers. Even in mucous membrane, which 

 ordinarily is not subject to mechanical forces of any intensity., 

 similar modifications are possible : witness the callosity of 

 the gums which arises in those who have lost their teeth, and 

 have to masticate without teeth. The vascular 



system furnishes good instances of the increased growth 

 that follows increased function. When, because of some 

 permanent obstruction to the circulation, the heart has to 

 exert a greater contractile force on the mass of blood which it 

 propels at each pulsation into the arteries, and when there re- 

 sults the laboured action known as palpitation ; there usually 

 occurs dilatation, or hypertrophy, or a mixture of the two : 

 the dilatation, which is a yielding of the heart's structure 

 under the increased strain, implying a failure to meet the 

 emergency ; but the hypertrophy, which consists in a thick- 

 ening of the heart's muscular walls, being an adaptation of it 

 to the additional effort required. Again, when an aneurism 

 in some considerable artery has been obliterated, either arti- 

 ficially or by a natural inflammatory process ; and when this 

 artery has consequently ceased to be a channel for the blood ; 

 some of the adjacent arteries which anastomose with, it, 

 become enlarged, so as to carry the needful quantity of blood 

 to the parts supplied. Though we have no direct 



proof of analogous modifications in nervous structures ; yet 

 indirect proof is given by the greater efficiency that fol- 

 lows greater activity. This is manifested alike in the 

 senses and the intellect. The palate may be cultivated in- 

 to extreme sensitiveness, as in professional tea-tasters. An 

 orchestral conductor gains by continual practice, an unusually 

 great ability to discriminate differences of sound. And in 

 the finger-reading of the blind, we have evidence that the 

 sense of touch may be brought by exercise to a far higher 

 capability than is ordinary. The increase of power which 



