1018 PHYSIOLOGY 



dependent on the difference of potential between the injured and uninjured 

 portion, is thus observed. If the vagus be now stimulated, the auricles 

 remain at rest but the demarcation current is increased, i. e. a positive 

 variation is produced an electrical condition opposed in sign to that which 

 would take place when the auricles contract. Doubt still exists however as 

 to the exact interpretation to be put on this experiment. 



It was mentioned above that potassium salts promote relaxation of the ventricle, 

 so acting as antagonists to calcium salts. If potassium salts be present in a sufficient 

 concentration in the circulating fluid, the heart is brought to a standstill in a condition 

 of diastole, as if the vagus mechanism were in action. On removal of the excess of K 

 ions, the heart at once starts beating again. Howell has shown that during stimula- 

 tion of the vagus the amount of potassium in a diffusible form in the heart muscle is 

 increased. He has therefore suggested that the action of the vagus in stopping the 

 heart is due to the liberation of potassium salts. Potassium normally exists in a 

 large percentage in the heart muscle, but in a combined form ; and Howell assumes 

 that stimulation of the vagus effects a dissociation of this combined potassium, so that 

 the liberated ions are able to exert their inhibitory influence on the heart. 



THE TONIC ACTION OF THE VAGUS 



If both vagi of a mammal be divided, the heart as a rule beats more 

 frequently, showing that under normal circumstances tonic impulses are 

 constantly descending the vagi and holding the heart's action in check. 

 The extent of the quickening, which is produced by section of the vagi, varies 

 in different animals and is apparently associated with the conditions of life 

 of the animal and its powers of carrying out prolonged muscular exertions. 

 Thus in the dog or horse the pulse, which is normally slow, may be doubled 

 in frequency by section of the vagi. In the rabbit, which has a frequent 

 pulse and is able to run only for a short distance, division of both vagi 

 causes very little alteration in the pulse rate. It is stated that the tonic 

 action of the vagi is much greater in the hare than in the rabbit. 



This tonic action may be increased by various conditions of the blood, 

 e. g. the presence of drugs such as morphia. 



ACTION OF THE SYMPATHETIC CARDIAC NERVES 



Stimulation of the sympathetic cardiac nerves at any part of their course 

 has an effect on the heart the exact reverse of that produced by stimulating 

 the vagi. In most cases the pulse frequency is increased in consequence 

 of the action of these nerves on that part of the heart from which the rhythm 

 starts. The frequency, which is attained by maximal stimulation of the 

 accelerator nerves, is independent of the previous rate of the heart beat. The 

 increase in rate involves a shortening of the time of the cardiac cycle, which 

 chiefly affects the diastolic period. The size of the auricular and ventricular 

 contractions may be increased at the same time as their rate. In fact, 

 like the vagus nerves, the sympathetic fibres of the heart can influence 

 rhythm, strength of contraction, or conduction from auricle to ventricle, 

 according to the part of the heart muscle which is affected. 



