1006 PHYSIOLOGY 



to the solution a trace of calcium chloride or calcium sulphate, the con- 

 tractions begin again (Fig. 449). The relaxations after each contraction 

 then become more and more incomplete, until finally the heart stops in a 

 tonically contracted condition. -If now a trace of potassium chloride or 

 phosphate be added, the contractions recommence and may last for many 

 hours, although the solution contains nothing which can furnish energy 

 to the contracting muscle. It has been suggested that the rhythmic con- 

 tractions of the heart muscle may be the result of the constant chemical 

 stimulus of the inorganic salts present in the blood plasma, sodium acting 

 as a stimulus to contraction, while the calcium salts are necessary for the 

 maintenance of the systolic tone, and the potassium salts for the occurrence 

 of relaxation. 



The exact significance of these different salts for the functions of cardiac 

 and other forms of muscular tissue, though they have been the subject of 

 many detailed investigations, must be still regarded as an open question. 



I 



FIG. 450. A frog's heart, poisoned by excess of calcium salts, recovers its spontaneous 

 rhythm on adding a trace of KC1 to the perfusion fluid. (RINGER). 



The fluids, containing the three salts mentioned above in slightly varying 

 proportions, are commonly used to maintain the beat in an excised heart 

 either of a cold- or of a warm-blooded animal. In the case of the latter it 

 is necessary to keep the fluid saturated with oxygen. According to Locke 

 the addition of glucose to the solutions enables the beats to go on for a 

 longer period of- time, and will in fact renew the rhythm of a heart which 

 has ceased beating while being fed with pure saline solution. 



The following represent the fluids most frequently used : 



RINGER'S FLUID 



(for frog's heart) 



1 per cent, sodium bicarbonate .... 1 c.c. 



1 calcium chloride . . . . . 1 c.c. 



1 potassium chloride .... 0-75 c.c. 



0*6 sodium chloride ... to 100 c.c. 



LOCKE'S FLUID 

 (for mammalian heart) 

 0-015 per cent, sodium bicarbonate, 

 0-024 calcium chloride, 

 0-042 potassium chloride, 

 0-92 sodium chloride, 

 0-1 glucose, 



in distilled water. 



The influence of the i-hemiral composition of the medium on the contraction of the 

 heart may be investigated in the following ways : 



