820 



SCIENCE. 



[N. S. Vol. V. No. 12R 



essential to know the nature of the dis- 

 turbances induced by the underlying morbid 

 condition, how these disturbances excite 

 increased functional activity of the part 

 which becomes hypertrophied, and what the 

 relationship is between this greater activity 

 and the increased growth of the part. 



It is impossible on this occasion to go 

 through the whole list of compensatory hy- 

 pertrophies with reference to the application 

 of these principles. In no instance can the 

 requirements stated be completely met in 

 the present state of our knowledge. It will 

 suflQce for an understanding of the principles 

 involved, and it is only with these that I 

 am now concerned, if I take a concrete ex- 

 ample. I select the classical and best studied 

 one — compensatory hypertrophy of the 

 heart. I trust that I shall be pardoned for se- 

 lecting so commonplace an illustration, as 

 the main points involved must be familiar to 

 most of my audience; but it is possible that 

 the application made of them may not be 

 equally familiar. The only matters essen- 

 tial to my present line of argument are the 

 mechanism of production of the hypertro- 

 phy and the general character of the adap- 

 tation thereby secured. 



The heart, like other organs of the body, 

 does not work ordinarily up to its full ca- 

 pacity, but it is capable of doing at least 

 three or four times its usual work. The 

 excess of energy brought into play in doing 

 this extra work is called conveniently, al- 

 though not without some impropriety, 

 'reserve force.' It has been proved ex- 

 perimentally that this storehouse of reserve 

 power is sufficient to enable the healthy 

 heart, at least that of a dog, to accommo- 

 date itself at once or after a few beats to 

 high degrees of insufficiency or obstruction 

 at its valvular orifices without alteration in 

 the mean pressure and speed of the blood 

 in the arteries. But even so tireless and 

 accommodating an organ as the heart can- 

 not be driven at such high pressure without 



sooner or later becoming fatigued, and con- 

 sequently so dilated as to fail to meet the 

 demands upon it. If it is to continue long: 

 the extra work, it must receive new incre- 

 ments of energy. 



The cardiac muscle is far less susceptible- 

 to fatigue than the skeletal muscles, but 

 that it may become fatigued seems to me 

 clear. 



Leaving out of consideration some doubt- 

 ful causes of cardiac hypertrophy, such as 

 nervous influences, the various morbid con- 

 ditions which lead to this affection are such 

 as increase either the volume of blood to be 

 expelled with each stroke or the resistance 

 to blood-flow caused by the pressure in the- 

 arteries or by narrowing at one of the 

 valvular orifices, or both. Unless some 

 regulating mechanism steps in, each of 

 these circulatory disturbances must in- 

 crease the resistance to contraction of the 

 cardiac muscle, and it is evident that the 

 heart must do extra work if it is to pump 

 the blood through the arteries with normal 

 pressure and speed. It is, however, no ex- 

 planation of this extra work simply to say 

 that it occurs because there is demand for 

 it. Increased work by the heart in cases 

 of disease of its nutrient arteries would 

 often meet a most urgent demand on the 

 part of the body, but here the heart flags- 

 and fails. 



The physiologists have given us at least 

 some insight into the mechanism by which 

 the heart responds through increased work 

 to the circulatorj^ disturbances which have 

 been mentioned. These disturbances all 

 increase the strain on the wall of one or 

 more of the cavities of the heart ; in other 

 words, increase the tension of the cardiac 

 muscle, in much the same way as a weight 

 augments the tension of a voluntary muscle. 

 Now it is a fundamental physiological law 

 that with a given stimulus greater tension 

 of a muscle, within limits, excites to more 

 powerful contraction, and thus to the per- 



