MECHANICAL RESPONSE OF CARDIAC MUSCLE. 441 



found that no further increase of intraventricular pressure could be 

 obtained, so that there appeared to be a certain initial impletion which 

 was most advantageous, and which could neither be exceeded nor 

 reduced without loss of effect. In skeletal muscle we measure the 

 " absolute force " by estimating the relation between the sectional area 

 of the contracting fibres and the weight which the muscle can just lift. 

 The same mode of estimation cannot be applied to the heart, because the 

 area of the cardiac fibres concerned in systole cannot be measured. 



It is probable that the product of the number and the mean sectional 

 areas of the fibres in the ventricle may be considered as proportional to 

 its size, so that we may express the absolute force of the ventricle 

 correctly by the maximum pressure which it is able to produce when 

 contracting against resistance. This can be easily got at by recording 

 the isometric curve in the way above indicated, in a series of experi- 

 ments in which the initial impletion of the ventricle is increased after 

 each observation, until the point is reached at which the maximum 

 pressure in question is attained. The observations can be made so quickly 

 that the results can be verified by repetition, without risk of exhausting 

 the organ {I.e. p. 386). 



The effect of after-loading. — It is not within the scope of this 

 article to deal with the mechanical relations of the circulation. It 

 must suffice to indicate the application of elementary principles derived 

 from the investigation of ordinary striped muscle, to the ventricle doing its 

 normal work. As we have already seen, the action of the ventricle 

 when contracting automatically does not differ in any respect from its 

 response to stimulation of the auricle by an induction shock. Both are 

 comparable to the response of a muscle when made to contract under 

 corresponding mechanical conditions. In the method of after-loading a 

 skeletal muscle, which is illustrated in Fig. 198, p. 368, we have seen that 

 the mechanical response to a single induction shock is divisible into three 

 periods, during the first and third of which the muscle is under isometric 

 conditions, while during the intervening period it does its work isotonic- 

 ally. The normal action of the ventricle is, as Frank shows, in a similar 

 manner divisible into three stages — those of distension, expulsion, and 

 relaxation, the first being separated from the second by the opening of 

 the aortic valve, the second from the third by its closure. During the 

 first period, contractile stress rapidly increases ; during the third, it more 

 gradually diminishes. During the intervening stage, that of* expulsion, the 

 relation between change of tension and change of volume differs accord- 

 ing to the lateral pressure, this being determined by the resistance in 

 front and the initial change. The effect of increasing the resistance in 

 front, in the case of the ventricle, corresponds to that of increasing the 

 load in the case of muscle. Empirical evidence of this correspondence was 

 obtained by Frank by comparing the curves of intraventricular pressure 

 in a series of experiments, in which the resistance in front was successively 

 increased by equal increments. In skeletal muscle the effects of increas- 

 ing the after-load are to defer the beginning of the isotonic period, to 

 shorten its duration, to diminish the velocity of the upward movement 

 of the free end of the muscle, and consequently to diminish the lift. In 

 the ventricle, the corresponding results of increasing the resistance in 

 front are to defer the beginning of the period of expulsion, i.e. the 

 opening of the aortic valve, to shorten the period between opening and 

 closure of the valve, and to diminish the rate of flow through the aortic 



