ADAPTATION. 229 



friction, is certain. Those who have not before exerted their 

 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 perma- 

 nent obstruction to the circulation, the heart has to exert a 

 greater contractile force on the mass of blood which it 

 propels at each pulsation, and when there results 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 in- 

 creased strain, implying a failure to meet the emergency; 

 but the hypertrophy, which consists in a thickening 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 artifically 

 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 follows greater 

 activity. This is manifested alike in the senses and the 

 intellect. The palate may be cultivated into extreme sensi- 

 tiveness, as in professional tea-tasters. An orchestral con- 

 ductor gains, by continual practice, an unusually great ability 

 to discriminate differences of sound. 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 ordi- 

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