THE MECHANISM OF THE HEART PUMP 1027 



its blood through 2a into cylinder II while cylinder I is emptying 

 itself through Iv into the right auricle. We thus get two series of zig- 

 zag lines traced by the piston rods, and the frequency of the zigzags is 

 an expression of the output of the left ventricle in a given time. 



This method suffers from the defect that the arterial pressure, in 

 consequence of the absence of external resistance, is extremely low, 

 so that the heart is throughout under highly abnormal conditions. It 

 enjoys, however, the corresponding advantage that R (the resistance), 

 though low, is constant throughout the experiment, and the work 

 done by the heart is therefore directly proportional to the output. 



In a method devised by the author it is possible to determine the 

 output of the left ventricle under more normal conditions, 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 respi- 

 ration being maintained, the chest is opened under an anaesthetic. 

 The arteries coming from the arch of the aorta in the cat, the inno- 

 minate and the left subclavian are then ligatured, thus cutting ofi 

 the whole blood-supply to the brain, so that the anaesthetic can be 

 discontinued. Cannulae are placed in the innominate artery and the 

 superior vena cava. The cannulae are filled beforehand with a solution 

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

 blood during the experiment. The descending aorta is closed by a liga- 

 ture. The only path for the blood left is by the ascending aorta and 

 the cannula Ca in the innominate artery. The arterial cannula 

 communicates by a T-tube with a mercurial manometer M ' to record 

 the mean arterial pressure, and passes to another T-tube, v, one limb 

 of which projects into a test-tube B. The air in this test-tube will be 

 compressed with a rise of pressure and will serve as a driving force 

 for the blood through the resistance. It thus takes the part of the 

 resilient arterial wall. The other limb of the T-tube passes to the 

 resistance R. This consists of a thin-walled rubber tube (e.g. a rubber 

 finger-stall) which passes through a wide glass tube provided with 

 two lateral tubulures w, w. One of these is connected with a mercurial 

 manometer M 2 and the other with an air reservoir A, into which air can 

 be pumped by the elastic bellows S. When air is injected into the outer 

 tube the tube R collapses, and will remain collapsed until the pressure 

 of the blood within it is equal or superior to the pressure in the air sur- 

 rounding it. It is thus possible to vary at will the resistance to the 

 outflow of the blood from the arterial side. From the peripheral end of 

 R the blood passes at a low pressure and is collected in a vessel N, which 

 is provided with a siphon, and can be made of such dimensions that the 

 blood is siphoned off as soon as 10, 20, or 30 c.c. have collected in the 

 vessel. A lateral branch on the siphon tube leads by a rubber tube 



