162 THE CIRCULATION OF THE BLOOD 



conditions must be fulfilled. The recognition of these conditions has 

 been accomplished by observations on the excised heart, for it has been 

 found that if they are fulfilled the mammalian heart can be made to beat 

 in perfectly normal fashion for several hours after its removal from 

 the animal's body. Indeed certain mammalian hearts, such as that of the 

 rabbit, may be made to beat for several days outside the body. We may 

 consider the essential conditions of the blood supply under four headings: 

 (1) the temperature; (2) the oxygen supply; (3) the pressure; and (4) 

 the chemical composition. Successful perfusion may be performed with 

 artificial saline solutions (e. g., Locke's), but it is simplest in investigating 

 the relative importance of the above conditions to start the heart per- 

 fusion with defibrinated blood. 



After bleeding. an anesthetized animal, such as a dog or a cat, until 

 no more blood can be removed, the blood is defibrinated and filtered 

 through gauze to remove the fibrin. The thorax of the dead animal is 

 then quickly opened, ligatures placed around the main arteries springing 

 from the arch of the aorta, a cannula with its end pointing toward the 

 heart inserted into the descending thoracic aorta, and the latter cut 

 across below the point of insertion of the cannula. The heart is then 

 quickly removed from the thorax and an artificial saline solution 

 (Locke's) allowed to run into the aortic cannula through a side tube, 

 until all the blood has been washed out from the coronary vessels. Dur- 

 ing this operation the heart may develop a few beats even though the 

 solution is quite cool. The aortic cannula is now connected with a bottle 

 containing the defibrinated blood diluted with Locke's solution and 

 brought to body temperature by immersion in a water-bath. By means 

 of a suitably regulated air pressure exerted on the surface of the diluted 

 blood in the bottle, this is forced through an outlet at the foot of the 

 bottle into tubing which runs to the aortic cannula. The fluid thus finds 

 its way into the coronary vessels; for in passing toward the heart in the 

 aorta it will close the semilunar valves and force its way under pressure 

 into the coronary vessels, subsequently escaping by the coronary sinus into 

 the right auricle. Very soon after the perfusion is started the heart 

 begins to beat vigorously and regularly, thus offering a suitable prepara- 

 tion upon which to test the first three mentioned conditions necessary 

 for the nutrition of the cardiac musculature (Fig. 39). 



If the temperature of the solution is allowed to fall considerably, the 

 beat becomes much slower, and if the cooling is proceeded with, the heart 

 will after a while cease beating altogether. If the pressure is lowered, 

 the beat will not necessarily become slower but very much feebler, and 

 will soon cease. In general it may be said that the temperature of the 

 solution affects the rate of the beat, and the pressure affects its strength. 



