426 VETERINARY PHYSIOLOGY 



tion dejoends upon the presence of certain electrolytes in the 

 circulating blood. A due admixture of salts of sodium, 

 potassium, and calcium is essential. For the frog's heart 

 Ringer found that the proportions which give the best results 

 are— 



NaCl . . . . 0-70 per cent. 



KCl .... 0-08 



CaCl .... 0025 



The mammalian heart may be kept contracting for a long 

 time after removal from the body by perfusing the coronary 

 arteries through a tube fixed in the aorta with a suitable 

 saline solution well oxygenated and kept at the temperature 

 of the animal. 



Sodium salts when supplied alone to the heart in con- 

 siderable amounts cause relaxation. Potassium salts in much 

 smaller amounts have the same effect. Calcium salts when 

 in excess cause a sustained contraction. 



The amount of carbon dioxide in the blood circulating in 

 the coronary system affects the action of the heart. A 

 decrease in the amount accelerates the heart, an increase 

 leads to increased diastolic relaxation ; but at first the con- 

 tractions are also so increased that the output of blood 

 remains unaltered. Later the contractions decrease in force 

 and the heart may stop in diastole. 



The intra-cardiac nervous mechanism seems to exercise 

 a controlling influence on cardiac contraction. (1 ) If the region 

 between the sinus and auricles in the frog's heart is stimu- 

 lated by the interrupted current from the induction coil, the 

 heart is slowed or stopped, even after the synapses with the 

 nerves coming from the central nervous system have been 

 poisoned by nicotine. If atropine, which poisons the terminal 

 plexus, be first applied, electric stimulation is without result 

 (Practical Physiology). 



(2) Further, if the intra-ventricular pressure in the heart of 

 the frog is raised by clamping the aorta, a slowing of the 

 rhythm occurs even after section of the vagi, but not after 

 the intra-cardiac neurons have been poisoned by atropine. 



