THE MECHANISM OF THE HEART PUMP 



911 



the ventricles are receiving blood and increase in volume, during systole 

 they expel blood and therefore diminish in volume. The change in volume 

 at each beat gives therefore the combined output of right and left ventricles 

 and must be divided by half in order to give the output of either ventricle 

 separately. 



METHODS OF DETERMINING OUTPUT. In a method devised by the author 

 it is possible to determine the output of the left ventricle under all manner of conditions 



FIG. 405. Arrangement of apparatus for~working on the isolated 

 mammalian heart. (KNOWLTON and STARLING.) 



and to vary at will the arterial resistance, the venous pressure, the filling of the heart, 

 or the temperature of the blood-supply to the heart. The arrangement of the apparatus 

 is shown in Fig. 405. Artificial respiration being maintained, the chest is opened under 

 an anaesthetic. The arteries coming from the arch of the aorta in the cat, the innomi- 

 nate and the left subclavian are then ligatured, thus cutting off the whole blood-supply 

 to the brain, so that the anaesthetic can be discontinued. Cannulse are placed in the inno- 

 minate artery and the superior vena cava. The cannulse are filled beforehand with a solu- 

 tion of hirudin in normal salt solution so as to prevent clotting of the blood during 

 the experiment. The descending aorta is closed by a ligature. The only path left for 

 the blood is by the ascending aorta and the cannula CA in the innominate artery. 



