THE CIRCULATION OF THE BLOOD 309 



98 to 102 per minute when it had a temperature of 4iC., though at this tem- 

 perature the beat became arrhythmic. All these instances demonstrate the 

 extreme persistence of the irritability of the heart-muscle under appropriate 

 conditions. 



The action of individual salts has been shown experimentally on the 

 hearts of rabbits, cats, dogs, monkeys, by Gross, Howell and others. Thus 

 it has been found that when an isolated heart is rhythmically beating in 

 response to the perfusion of Ringer's or Locke's solution, the addition of 

 potassium chlorid in small amounts is followed by a decrease in the rate and 

 force of the contraction, and in larger amounts by a complete cessation of the 

 contraction and a standstill in diastole. On the withdrawal of the potassium, 

 the former frequency and vigor are regained. 



Under the same conditions, the addition of calcium chlorid in sufficient 

 amounts is followed by an increase in the rate and in the vigor of the 

 contractions; on its withdrawal both rate and force return to the previous 

 condition. Potassium exerts a depressor or an inhibitor influence on the 

 irritability and contractility of the heart-muscle. Calcium exerts an accel- 

 erator and an augmentor influence on the irritability and contractility of the 

 heart. 



A Theory of the Heart-beat. From the foregoing facts it seems 

 probable that the heart-beat is connected with and dependent on the presence 

 and interaction of the inorganic salts present in the lymph, though as to the 

 manner in which they interact to initiate the beat, there is some obscurity. 

 A very plausible theory as to the part played by the inorganic salts in initiat- 

 ing the contraction and one apparantly in accordance with the facts has been 

 presented by Howell as follows: 



The heart-muscle, it is assumed, contains a stable organic energy-yielding 

 compound of which potassium is one of the constituents and on which its 

 stability depends. This compound must be present in relatively large 

 amounts as the heart will continue to contract and expend energy for many 

 hours after the blood-supply has been withdrawn. 



During the diastole a reaction takes place between this compound and 

 the calcium or the calcium and the sodium salts, whereby a portion of the 

 organic compound is freed from potassium and is then combined with calcium 

 or with calcium and sodium. In consequence, this portion of the organic 

 compound in combination with the calcium acquires and gradually increases 

 in instability, reaching its maximum at the end of the diastole, when it under- 

 goes a dissociation giving rise to a chain of events that culminate in a con- 

 traction. The initial step, therefore, is a dissociation of a complex unstable 

 molecule followed by an oxidation of the dissociated products. That an 

 active dissociation of some character takes place is evident from the consump- 

 tion of oxygen, the production of carbon dioxid, the liberation of heat, 

 electricity, and mechanic motion. 



Inasmuch as the contraction is always maximal and as the heart is refrac- 

 tory to a stimulus during the systole, the probabilities are that all of the 

 unstable portion of the energy-yielding compound is dissociated with each 

 contraction. With the relaxation there is a renewal of the unstable 

 combination of calcium with the organic molecules, which increases 

 in amount until the maximum is again attained when another dis- 

 sociation occurs followed by another contraction. The rhythmicity of 



