300 TEXT-BOOK OF PHYSIOLOGY, 



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 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 hours after the blood-supply has been withdrawn.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 

 the heart's action, the appearance of a refractory condition during the 

 systole and its gradual disappearance during the diastole, as well as other 

 phenomena, are readily explained by the foregoing hypothesis. 



The cause of the dissociation of the energy-yielding material is, however, 

 a subject of discussion. According to Howell it is not necessary to assume 

 the presence of any cause other than the extreme instability of the organic 

 compound in question. According to Engelmann, Langendorff and others, 

 the dissociation is not spontaneous but is the result of the action of a specific 

 stimulus, an "inner stimulus," arising within the muscle elements themselves 

 through metabolic processes; and so long as these processes are chemically 

 and physically conditioned by blood or tissue fluids containing the inorganic 

 salts, so long will this stimulus be produced. As to the nature of this stimu- 

 lus, whether chemic, electric or enzymic, nothing definite can be stated at 

 present. 



The Response of the Heart to the Action of an Artificial Stimulus. 

 The heart of the frog as well as of some other animals may be brought to a 

 standstill by the ligation of the tissues between the sinus venosus and the 

 auricle, a procedure first introduced by Stannius and now known as the first 



