374 Professor E. H. Starling [May 20, 



the third stage the artificial resistance was suddenly reduced to zero, 

 the arterial pressure fell to about 20 mm. Hg., but the heart beat 

 regularly and the outflow of blood was unaltered because the inflow 

 of blood had not been altered. In the fourth stage the inflow of 

 blood was raised suddenly to 600 cc. per minute. The heart became 

 bigger, but the regularity of its contractions remained unaltered, 

 and it drove forward all the blood that it received. 



The same thing happened in the fifth stage, in which the artificial 

 resistance was raised simultaneously with the venous inflow. The 

 reason for these phenomena is that within certain limits the heart 

 isolated from the body can respond to all the demands made upon 

 it ; it can overcome a higher resistance, and it can pump out more 

 fluid. In the sixth stage the inflow of blood was further increased 

 to 1200 cc. per minute, and the artificial resistance was increased 

 until the blood pressure rose to 200 mm. Hg. This was too much 

 for the heart, which began to beat irregularly and dilate widely. It 

 would have failed altogether if the pressure surrounding the thin 

 rubber tube had not then been released, to allow the artificial pressure 

 to drop to a level at which the left ventricle could empty itself. 



If during this experiment the amount of oxygen taken up by the 

 blood had been measured, and also the amount of carbonic acid 

 given off by this fluid in passing through the lungs, an increase in 

 both these amounts would have been found during the stage at 

 which greater demands were being made on the heart. That is to 

 say, the greater the work done by the heart, the greater the chemical 

 changes to supply energy. A motor-car may be running steadily 

 with an even beat of its engines along a level road ; when it comes 

 to a hill it will slow up and finally stop unless the chauffeur increases 

 the chemical changes and the energy of each explosion within the 

 cylinder by opening the throttle and letting in more mixture of 

 petrol and air. In the case of the heart there is no chauffeur, but 

 there is some automatic regulation by which the heart increases its 

 chemical changes, and therefore the energy of each beat, in exact 

 proportion to the work which is demanded of it. It is the nature of 

 this automatic regulation which concerns us now. 



The Nature of the Automatic Regulation of the Heart. 



By a careful observation of the changes in the heart in the 

 experiment described above we may arrive at some clue to the nature 

 of the pressure, but more accurate methods are necessary if we are 

 to be certain of the correctness of our guess. We must under these 

 varying conditions, measure : (1) the pressure in the heart cavities 

 produced at each contraction ; (2) the volume of the heart cavities — 

 i.e. the length of the muscle fibres of their walls. The first we 

 measured in the experiment described by connecting the interior of 

 each cavity in turn with a quickly acting manometer, the excursions 



