272 THE HUMAN BODY. 



it is possible to keep it alive for some hours, beating regularly 

 and powerfully, and on such a heart it is easy to observe that 

 cooler blood causes slower beat and vice versa. While the 

 quick pulse observed in fevers may therefore be in part due 

 to paralysis of the cardio-inhibitory centre or stimulation 

 of the cardio-accelerator, it is in part at least due solely to 

 the hotter blood circulating through the coronary vessels. 

 Whether the higher temperature in this case acts primarily 

 on the nerve-cells of the heart or on the muscle is not known. 

 If circulation be kept up through a frog's heart by the 

 perf usion method (Fig. 100), the organ may be kept beating for 

 a very long time if the liquid supplied be blood or serum. If 

 only dilute solution (0.75$) of sodium chloride be given, the 

 beat continues for some time, but not so long as if no liquid 

 be circulated ; the salt apparently washes out something 

 which the heart needs. The beat of such a " washed-out " 

 heart may be restored by substituting milk or serum or de- 

 fibrinated blood for the saline solution, or even by adding to 

 the sodium chloride a very little of a soluble calcium salt. 

 Serum, blood, and milk all contain calcium salts, and albu- 

 minous solutions free from calcium (as paraglobulin) do not 

 restore the beat; nor do serum or milk or blood deprived of 

 calcium. Hence the presence of some salt of that metal 

 seems to have a close relation to the functional activity of 

 the heart, as indeed it has to muscular activity in general. 



