262 PHYSIOLOGY OF THE HEART [CH. XXI. 



2. It must circulate through the coronary vessels. 



3. It must be well oxygenated. 



As before, living blood is the ideal fluid for perfusion, but the 

 practical difficulties in its use are so great, that a modification of 

 Einger's fluid is usually employed. On this fluid the heart will 

 continue to beat for many hours, but it will beat longer (sometimes 

 several days) if a little dextrose is added to the solution. We owe 

 this addition, and the oxygenation alluded to above, to Dr Locke ; 

 and the perfusion fluid now universally employed is consequently 

 called Locke's solution. This has the following composition: 



Pure distilled water . . . . 100 c.c. 



Sodium chloride . . . . . 0'9 grammes. 



Potassium chloride . . . . . 0'042 



Calcium chloride . . . 0'048 



Sodium bicarbonate . . . . 0'02 ,, 



Dextrose . . 0'2 



Dr Locke has tried other sugars besides dextrose, but no other 

 has the same favourable effect ; Isevulose is better than most other 

 sugars, but not nearly so good as dextrose. Locke and Rosenheim 

 have further shown that the dextrose is used up during cardiac 

 activity, and this lends support to the view already expressed on the 

 importance of this kind of sugar as a source of muscular energy 

 (see p. 131). 



A mammal such as a cat or rabbit is killed by bleeding or 

 pithing. The heart enclosed in the pericardium is quickly cut out, 

 and gently kneaded to free it from blood, in some warm Einger's 

 solution. The pericardium is then dissected off, and a cannula tied 

 into the aorta; this is connected to a burette which is kept full of 

 Locke's solution. The solution must be maintained at body tempera- 

 ture, by a warm water-jacket, and must be well oxygenated by letting 

 oxygen bubble through it. The fluid is then allowed to flow, and its 

 pressure closes the aortic valves, and so the fluid enters the coronary 

 arteries, and escapes from the right auricle, which should be freely 

 opened. Under these circumstances the heart will continue to beat 

 for many hours. A graphic record may be obtained by putting a 

 small hook into the apex, and attaching this by a thread to a record- 

 ing lever beneath it. A very good illustration of the usefulness of 

 the method for demonstrating the action of drugs consists in adding 

 a small amount of chloroform to the circulating fluid, and one notices 

 its immediate depressant effect ; on the other hand, a minute dose of 

 adrenaline markedly increases the rate and force of the heart. 



