1921] on the Law of the Heart 875 



of which are registered by an optical method so as to avoid the 

 instrumental vibrations of a lever. The curve of pressure obtained 

 under two conditions— i.e. low and high artificial resistance — could 

 then be plotted. It must be remembered that the heart was sending 

 on in each case all the blood that it received, though the work 

 necessary under the high pressure was two or three times as great as 

 that necessary to send on the blood at the low pressure. To measure 

 the volume of the heart the ventricles are enclosed in an instrument 

 known as a cardiometer. This communicates with a piston recorder 

 so that the change of the volume of the ventricles at each beat can 

 be registered on a moving surface. 



The question we have to decide is, How does the heart know 

 when it is relaxed that at the next contraction it will have to exert 

 more force than it did previously, when the arterial resistance to be 

 overcome was lower ? If we measure the pressure in the ventricles 

 in the manner just described we find that during the period of 

 relaxation of the ventricles, the pressure in its cavities is approxi- 

 mately zero, whether the artificial pressure which it has to overcome 

 at its next beat is 50 or 150 mm. Hg. It is not, therefore, the 

 tension on the walls of the heart which determines the strength of 

 its contraction at its next beat. When, however, we come to measure 

 the volume of the heart, we find that in the isolated heart this is 

 directly proportioned to the work which the heart has to accomplish. 

 Thus we find that the larger the heart — i.e. the more it is dilated 

 during diastole — the greater is the pressure that it will get up at the 

 succeeding contraction or systole. 



AVe may put this in another form, as is shown by continuing our 

 experiment over several hours, when we find that the worse the con- 

 dition of the heart muscle, the more it must dilate in order to get up 

 an adequate pressure. Other things remaining equal, we thus see 

 that the volume of the heart during diastole is a measure of its 

 physiological condition, and we are not surprised that a failing heart 

 means a dilated heart. Of course there is a limit to this power of 

 adaptation. As the heart dilates it is working at an ever-increasing 

 mechanical disadvantage, and a point will finally arrive at which this 

 disadvantage more than counterbalances the physiological effect of 

 dilatation. The heart then dilates widely and fails to empty its 

 contents. Dilatation of the heart means elongation of the muscular 

 fibres composing its walls, so that we may put the law of the heart 

 another way and say that the longer its muscle fibres the greater is 

 the energy developed at each contraction. But in this form this 

 wonderful power of adaptation possessed by the heart becomes part 

 of the general properties of all muscular tissues, since the same rule 

 applies to the fibres composing our voluntary muscles. Can we 

 obtain any more precise and physiological conception of what is 

 involved in this relationship between length of fibre and strength of 

 contraction ? Microscopic examination of the fibres, either of the 



