174 THE CIRCULATION OF THE BLQOD 



responsive to the proportions of these salts in the blood, or in any artificial 

 solution which may be substituted for blood. If the rhythm is to be taken 

 as an index of the irritability, then an increase of sodium and calcium salts 

 increases the irritability (rhythm), while the influence of an increase in potas- 

 sium is to depress the irritability (rhythm). 



Cardiac Contractions Always Maximal. The heart muscle exhibits 

 another property which distinguishes it from ordinary skeletal muscle, viz., the 

 way in which it reacts to stimuli. The latter, Chapter XIII, reacts slightly 

 to a stimulus little above the minimal, and with an increase of the strength 

 of the stimulus will give contractions of increasing amplitude until the maxi- 

 mum contraction is reached. In the case of the heart-beats this is not so, 



FIG. 174. Refractory Period in the Ventricular Strip of the Terrapin. 



since the minimal stimulus which has any effect is followed by the maximum 

 contraction; in other words, the weakest effectual stimulus brings out as 

 great a contraction as the strongest. If a contraction is induced earlier than 

 it would automatically occur, then the succeeding pause is longer, i.e., there 

 is a compensatory pause. Also the contraction induced is smaller and the 

 one following the compensatory pause is proportionately larger. This ob- 

 servation can easily be demonstrated on the heart strip, see figure 174, or on 

 the whole ventricle of the frog, which was originally used by Bowditch. 



Nerve influence, nutrition, temperature, etc., will of course affect 

 the extent of the contractions, but under a given set of conditions it is held 

 that the contractions which occur are maximal for the particular state. This 

 is more readily understood when taken in connection with the fact that when 

 a contraction originates in a cardiac cell it is conducted throughout the ex- 

 tent of all the cells of the muscular mass. 



Theories of the Heart-Beat. The cause of the rhythmic power 

 of the heart as a whole has been the subject of much discussion and experi- 

 mental observation. Two leading hypotheses have given inspiration to 



