THE PRESSURE AND WORK OE THE HEART. 41 



curvature of the ventricle have become less, the action of these muscles 

 is no longer needed. 1 



The total stress in a dilated heart will be far greater than in a 

 contracted heart. It is for this reason advantageous that the diastolic 

 pressure of the heart be, by the mechanism of the veins, kept very low 

 and fairly constant. If, during diastole, the heart be too forcibly 

 distended, it may in this distended state be quite unable to contract. 

 This condition is seen in asphyxia and in chloroform poisoning. It can 

 readily be produced by pushing chloroform to excess, and by followino- 

 this with forcible compression of the abdomen with the hand, so that 

 the abdominal veins empty into the right heart. On relieving the 

 distension by letting blood, the right ventricle will begin to beat once 

 more. 



It is found on examination that the heart when dilated does not 

 empty ; the residual blood is seen to increase on vagal inhibition, on 

 compression of the aorta, on abdominal compression, or on injection of 

 fluid into the veins. 2 



The force of the heart is increased by flushing the coronary arteries 

 with blood, and improving the nutrition. If a maximal manometer 

 be placed in the right ventricle, and the pulmonary artery be 

 temporarily occluded, the maximal tension exerted by the wall of 

 the ventricular cavity can then be estimated. If, after making such an 

 observation, the aorta be temporarily occluded so as to increase the 

 blood supply of the coronary arteries, and the maximal pressure in the 

 right ventricle be again estimated, it will be found to be higher. Thus 

 a rise of aortic pressure may increase the force of the heart, and this 

 may even more than counterbalance the increase of work demanded. 3 



Tension also seems to increase the power of muscle, and thus a 

 physiological advantage may compensate for a physical disadvantage. 

 The effect of tension is seen in the case of the arteries. The arteries 

 react to tension with a force that is greater than the force that distends 

 them. Similarly, the power of the isometric muscle preparation when 

 contracting against the pull of a steel spring is greater than that of a 

 muscle which is after-weighted, and not in a state of tension at the 

 moment of excitation. Likewise, the apex preparation of a frog's heart 

 can be made to beat by raising the intracardiac tension ; the inhibitory 

 power of the vagus can be diminished by raising the tension of the 

 blood within the chambers of the heart ; the beat of the snail's heart 

 can often be arrested by merely letting the blood escape from 

 within it. 



In the case of the skeletal muscles, Maggiore 4 has shown that 

 there is a definite weight and rate of contraction essential to the 

 execution of the greatest amount of work in a given time. Either in- 

 creasing the weight or the rate of contraction lessens the total output 

 of work, and hastens fatigue. With a given light weight, the abductor 

 indicis can be made to contract and raise this weight once a second 

 for two and a half hours without any sign of fatigue. On increasing 

 the rate of contraction, or the weight to be lifted, fatigue rapidly super- 



1 Woods, Journ. Anat. and Physiol., London, 1891-2, vol. xxvi. p. 362; Samways, 

 Brit. Med. Journ.. London, 1897, vol. i. p. 703. 



2 Roy and Adami, Phil. Trans., London, 1892, vol. clxxxiii. B, p. 210. 



3 Ibid., p. 269. 



i Arch.f. Physiol., Leipzig, 1890, S. 191. 



