THE CAUSATION OF THE HEART-BEAT 961 



Hg., or whether it is receiving 5 c.c. or 10 c.c. during the period of diastole 

 With a bigger outflow or a bigger resistance the energy of contraction is 

 increased, although the tension on the heart wall at the beginning of the 

 contraction is not altered. The only condition then which always changes 

 pari passu with the energy of contraction is the distension of the heart 

 cavities, i.e. the length of its muscle fibres, and we are therefore justified 

 in regarding this last factor as the one which determines the energy of the 

 response of the muscle to excitation. Naturally if the distension increased 

 beyond a certain extent it would be associated with increased tension on the 

 muscle fibres. But the changes of initial tension and excitatory response 

 are not proportional. It is evident that the capacity of the heart for adapting 

 itself to changes in mechanical demands made upon it will be limited by the 

 inability of the heart to dilate further, as is probably the case in the intact 

 animal, where its dilatation is limited by the pericardium, or by the mechani- 

 cal disadvantage at which the further dilated heart acts. The more the 

 heart attains a globular form, the greater the mechanical disadvantage of 

 the muscle fibres in raising the pressure in the interior of the ventricles (vide 

 p. 916) so that by continually increasing the demands on the heart we shall 

 finally arrive at a stage at which this organ is unable to deal with the blood 

 which is applied to it and rapidly fails to expel any of its contents. 



The physiological condition of the heart is measured by the maximum 

 pressure which it is able to produce in its cavities when it contracts, starting 

 from a certain initial size or length of fibres. As the heart becomes fatigued 

 this pressure falls so that the heart must dilate in order that each contraction 

 shall produce the same maximum pressure as before. Fatigue of the heart is 

 shown therefore, not by failure of it to do its work, but by the fact that 

 it can only do its work when it is undergoing considerable dilatation. 

 Dilatation is therefore a measure of fatigue. What is often spoken of as the 

 tonus of the heart is really synonymous with physiological condition. A 

 heart in good condition has a high tonus. It empties itself almost completely 

 at each beat even when receiving a considerable quantity of blood during 

 diastole. A heart with a low tonus is in the condition of a fatigued heart. 

 It is widely dilated and when it has finished contracting still contains a large 

 amount of residual blood. 



This property of the cardiac muscle is responsible for the power of 

 ' compensation ' possessed by a diseased heart. We may take as an example 

 the destruction of one aortic valve, a lesion which can be produced experi- 

 mentally in a dog. In this case, immediately after the lesion is established, 

 no additional resistance is offered to the expulsion of the blood, and the ven- 

 tricle will send the normal amount into the aorta. During the succeeding 

 diastole the blood at a high pressure in the aorta will leak back into the 

 ventricle through the damaged valve. The arterial pressure therefore falls 

 rapidly, and the ventricle receives blood from two sides, i.e. by regurgitation 

 through the aortic valves, and in the normal way from the auricles and veins. 

 At the end of diastole the ventricle is therefore overfilled. Increased 

 stretching of its fibres, however, has the effect of exciting an increased con- 



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