194 THE CIRCULATION OF THE BLOOD 



stimulus applied at any time from the beginning of the contraction until 

 the summit of the contraction is reached. This is called the refractory 

 period. From the summit, through the relaxation and succeeding pause, 

 the irritability rapidly increases until the beginning of the next contraction. 

 Considering the automatically contracting muscle, the point in which the 

 automatic contraction is released, i.e., begins, is the point of maximal irrita- 

 bility. It is the moment when the irritability is so great that the muscular 

 equilibrium is no longer stable, and the physiological contraction results. 



The irritability of heart muscle is very sharply influenced by its condition 

 of nutrition, especially by the inorganic salts present in the blood and lymph, 

 see page 199. The salt content of the blood comprises about 0.7 per cent, 

 sodium chloride, 0.03 per cent, potassium chloride, and 0.025 to 0.03 per 

 cent, calcium (phosphate probably), as well as traces of other metal bases. 



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



The heart muscle has been shown by numerous investigators to be delicately 

 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. 



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 maximum contraction is reached. In the case of the heart-beats this is 

 not so, since the minimal stimulus which has any effect is followed by the maxi- 

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

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



