66 THE HEART 



tube passes to a resistance R. This consists of a thin-walled tube 

 (e. (J. a rubber finger-stall) which passes through a wide glass tube 

 provided with two lateral tubulures u\, tv^. One of these is con- 

 nected with a mercurial manometer, M2, and the other with an air 

 reservoir into which air can be pumped. When air is injected 

 into the outer tube, the tube B coUapses, and will remain collapsed 

 until the pressure of the blood within it is equal or superior to the 

 pressure in the air surrounding it. It is thus possible to vary at 

 will tlie resistance to the outflow of the blood from the arterial 

 side. From the peripheral end of R, blood passes at a low pressure 

 through a spiral immersed in warm water, into a large glass reservoir. 

 From the reservoir a wide india-rubber tube leads to a cannula 

 which .is placed in the superior vena cava, SVC, all the branches 

 of which have been tied. This cannula is provided with a ther- 

 mometer to show the temperature of the blood supplied to the 

 heart. A tube placed in the inferior vena cava and connected 

 with a water manometer shows the pressure in the light auricle. 

 On the recording suiface we thus have a record of the arterial 

 pressure and of the pressure within the right auricle. The output 

 of the whole system can be measured at any time by opening the 

 tube X, clamping Y, and allowing the blood to flow for a given 

 numljer of seconds into a graduated cylinder. . . . 



" The output . . . represents the ventricular output minus the 

 blood-flow through the coronary arteries. It is possible, however, 

 to insei't a cannula into the coronary sinus, and so to measure the 

 blood-flow through the heart-muscle." Artificial respiration is 

 continued throughout the experiment. 



The volume of the heart is measured by means of a 

 cardiometer, a glass vessel which encloses the organ. By 

 a side tube it is connected with a tambour, the movement 

 of which is recorded on a drum. 



The oxygen consumption of the heart is estimated from 

 the oxygen absorbed by the lungs. 



We may now briefly discuss how the output is affected by 

 changing any of the conditions. 



1. Temperature of the Blood. — The beat increases in rate 

 with rise of temperature. 



2. Reaction of the Blood. — At a certain reaction of the 

 blood, the output of the heart is maximum. Slight increa.se 



