THE NERVOUS REGULATION OF THE HEART 1093 



inhibitory process. According to that put forward by Claude Bernard, 

 the natural tendency of any tissue during rest is towards anabolism. 

 Activity involves disintegration or breaking down of the living material, 

 and this disintegration must be succeeded by a process of building up 

 or anabolism, which restores the tissue to its previous functional con- 

 dition. On this view the state of inhibition would merely prolong the 

 period of rest intervening between two periods of activity, so allowing 

 a greater time for restitution to take place, with a corresponding 

 improvement in the functional capacity of the tissue. According to 

 Hering and Gaskell a state of anabolism can be induced in a tissue com- 

 parable to the state of sudden disintegration which is associated with 

 activity. Excitation of the vagus nerve does not merely allow the 

 normal process of building up, which goes on during rest, to take place, 

 but actually hastens these processes, just as the excitation of a motor 

 nerve to a skeletal muscle induces an active breakdown of the con- 

 tractile tissue, or the excitation of the augmentor nerve to the heart 

 induces an increased rate of beat and therefore increased functional 

 activity. 



If stimulation of an inhibitory nerve induces the opposite chemical 

 change to that occurring during activity, one would expect to find 

 that, just as an active part of a tissue is negative to an inactive part, 

 so a part of the tissue which is under the influence of an inhibitory 

 stimulus should be electio-positive to any part which is not being so 

 stimulated. According to Gaskell this condition is realised in the heart 

 of the tortoise. The auricles are brought to a standstill by separating 

 them from the sinus venosus. The apex of one auricle is then injured by 

 heat, and the injured point and uninjured base are led off to a galvano- 

 meter. The usual demarcation current 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 inactive. 

 On removal of the excess of K' ions the heart at once starts beating again. Howell 

 has shown that during stimulation 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 effected by 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. 



