CARDIAC CONTRACTIONS 199 



not be occurring at the same time the ordinary fundamental rhythmic 

 contractions, figure 170. 



Irritability of Heart Muscle. Mention was made above of the difference 

 in irritability of heart muscle chosen from different parts of the heart. 

 The irritability of the muscle of each part also varies during the different 

 stages of the contraction. Experiment shows that the muscle is not irri- 

 table to a stimulus applied at any time from the beginning of the contrac- 

 tion until the summit of the contraction is reached. This is called the 

 refractory period. From the summit, through the relaxation and succeed- 

 ing 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 irritability. It is the moment when the irritability is 

 so great that the muscular equilibrium is no longer stable, and the physio- 

 logical 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 207. 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 



