HiL' THE CIRCULATION OF THE BL06t> 



conditions must be fulfilled. The recognition of these conditions lias 

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

 Pound thai if they are fulfilled tlie mammalian heart can be made to beat 

 in perfectly normal fashion for several hours after its removal from 



the animal's body. Indeed certain ma iiima I ia 11 hearts, such as that of the 

 rabbit, may be made to beat for Several days outside; the body. We ma\ 

 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 com posit ion. 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 pel' 

 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 lube, 

 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 

 broughl 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 tin 1 fool of the 

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



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



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



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



the righl auricle. Yvvy soon after the perfusion is started the heart 



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

 tion upon which to lest the first three mentioned conditions necessary 

 for the nutrition id' the cardiac inuscnlat lire (Fig. 39). 



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

 heat 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 mil necessarily become slower but \\-vy 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, 



