1921] on the Law of the Heart 373 



The Heart- Lung Preparation. 



All these procedures and controls can be carried out in the heart- 

 lung preparation. In this preparation the pulmonary circulation 

 from right ventricle to left auricle is left intact, and by means of 

 artificial respiration the lungs are blown up rhythmically so that the 

 blood in its course may take up oxygen and get rid of carbonic acid. 

 The whole systemic circulation is replaced by rubber tubes. A glass 

 tube is tied into the largest branch of the aorta, all the other 

 branches being tied, so that the blood driven out by the left ventricle 

 can only escape by the glass tube. From the glass tube a rubber 

 tube passes to a thin rubber tube contained within a wide glass tube. 

 This thin rubber tube can be compressed to any desired extent by 

 pumping air at a known pressure into the glass tube surrounding it. 

 We can thus vary at pleasure the resistance which has to be over- 

 come by the left ventricle, and, by maintaining a normal pressure in 

 the beginning of the aorta, ensure a proper supply of oxygenated 

 blood through the coronary arteries to the muscular tissue of the 

 ventricles. It is this fact which makes it possible for the warm- 

 blooded heart to continue to beat for eight to twelve hours after 

 removal from the body. On the other side of the artificial resist- 

 ance the blood is led through a spiral immersed in warm water to 

 keep the blood at body temperature, and then passes into a reservoir 

 from which a wide rubber tube leads to a glass tube placed in the 

 big vein opening into the right auricle. By means of a screw clip 

 on this tube the inflow of blood may be regulated to any desired 

 extent, and can be kept constant while other conditions are varied. 

 Thus in this preparation the three chief factors, temperature, the 

 inflow of blood, and the resistance to the outflow of blood, can be 

 varied separately and at the will of the operator. Any of the heart 

 cavities or any part of the circuit can be connected to manometers 

 so as to record the pressure of the fluid, and by means of a side tube 

 placed just beyond the artificial resistance we can allow the blood to 

 flow off into a graduated cylinder, and thus measure the time taken 

 by the left ventricle to expel 50 or 100 cc. of blood, thus measuring 

 the average output of the organ. 



An Experiment Described. 



A typical experiment may be divided into six stages. Records 

 of one experiment show that in the first stage the heart was beating 

 at a normal rate (72 per minute), the blood pressure varied from 

 100 mm. Hg., and the output of the heart was 240 cc. of blood per 

 minute. In the second stage the resistance to the flow of blood 

 through the tubes was increased to such an extent that the pressure 

 rose to 160 to 180 mm. Hg. The heart continued to beat, and for a 

 time put out just as much blood as it did at the lower pressure. In 



