159 



freezing the animal. The nerves of the excised heart retain their 

 action for some time if the nutritive fluid is immediately circulated 

 through the coronary arteries. Thus, the heart's action can be con- 

 veniently studied when taken from the body of a mammal. 



By using defibrinated blood mixed with the perfusion fluid food 

 material is brought to the heart. By some it is considered that the 

 serum albumin is the essential substance. In Locke's fluid* dextrose 

 is added to the above salts, and this forms an admirable perfusion 

 fluid. But the heart will beat when supplied with oxygenated Ringer's 

 solution only ; neither the serum albumin nor the dextrose appear to 

 be necessary. The important factors are the free ions of sodium, potas- 

 sium, and calcium, and certain concentrations of these appear to be 

 absolutely necessary for the rhythmic automatlcity and efficient 

 working of the heart. With increase of the contents of the calcium 

 ions the heart contracts more powerfully (Fig. 64). If the amount 

 of calcium be large the heart relaxes less and less completely, and 

 eventually stops in a state of tonic contraction. 



FIG. 6-1. ISOLATED RABBIT'S HEART PERFUSED WITH LOCKE'S SOLUTION. (Dixon.) 

 At the arrow 5 mgrm. of calcium chloride were given. Time in seconds. 



Excess of the potassium ions has an opposite effect (Fig. 65). The beat 

 of the heart becomes more and more feeble, and it ceases to beat in a 

 state of complete relaxation (diastole). Excess of the sodium ion 

 causes the beat of the heart to become weaker and weaker, and 

 eventually fail altogether (also in diastole). There is therefore an 

 antagonism between the calcium ions and those of sodium and 

 potassium. 



The origin of the excitatory wave is intimately dependent upon 

 interaction between these ions and the colloids of the heart muscle, 

 those of calcium playing a prominent part in the contraction of the 

 heart, those of potassium in its relaxation. 



If the heart is treated with lactic acid until it is brought to a 

 standstill in diastole, it can be partly restored by increasing the con- 

 centration of calcium in the Ringer's solution, and completely restored 



* Locke's fluid is distilled water, 100 c.c. ; sodium chloride, 0-9 gramme; potassium 

 chloride, 0-042 gramme; calcium chloride, 0-048 gramme; sodium bicarbonate, 0-02 

 gramme ; dextrose, 0-2 gramme. 



