THE PHYSIOLOGY OF THE HEART 



129 



to the stimuli characteristic for muscle (ammonia, dilute mineral 

 acids), and not to those for nerve (glycerine). We see, then, that the 

 heart possesses the properties of excitability and contractility, and 

 that excitability (the power to respond to a stimulus from without) 

 continues longer than the property of rhythmic automaticity (the 

 power of responding to the stimulus given from within). 



Thus the heart of the embryo chick at three days possesses great 

 rhythmic automatic power, but little excitability to artificial stimuli; 

 later, the rhythmic automaticity, especially that of the ventricles, 

 decreases, while the excitability increases. The auricle becomes 

 more automatic than the ventricle, the ventricle more excitable than 

 the auricle. 



The power of contractility varies in different hearts and in 

 different parts of the heart. It is most marked in the ventricular 

 musculature. The difference between automaticity and excitability 



TIG. 37. EFFECT OF INCREASING DIASTOLIC PRESSURES THAT is, INCREASING THE 

 LOAD ON THE ISOMETRIC CURVE OF THE FROG'S HEART. (0. Frank.) 



is more apparent than real, automaticity depending on the site of 

 application and the kind of stimulus. 



The contraction obtained by stimulating cardiac muscle resembles 

 that obtained by stimulating ordinary muscle (see Fig. 275). Just as 

 a striped muscle works better with increasing load up to a certain 

 point, so, tco, the heart-muscle contracts more powerfully as the 

 load is increased up to a certain point, but after that weakens and 

 stretches (Fig. 37). In severe muscular exercise the work of the 

 heart is greatly increased: contracting more often, it rests less. The. 

 heart responds to such strains 'by growing larger and stronger. 



Cardiac muscle presents certain fundamental characteristics which 

 are different from those of striped muscle. Thus, cardiac muscle 

 responds to any efficient stimulus with a maximal contraction, some- 

 times called the "all-or-nothing law." Striped muscle responds-ttr 

 increasing stimuli by contractions rising from minimal to maximal. 

 The difference is more apparent than real, for probably only a few of 

 the fibres of a muscle are stimulated by a minimal stimulus, while in 

 the syncytium of the heart-muscle the stimulus spreads everywhere. 



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