156 



THE CIRCULATION OF THE BLOOD AND LYMPH 



The refractory period is shorter for strong than for weak stimuli, 

 and is markedly diminished by raising the temperature of the heart. 

 So that stimulation of the heated heart with a series of strong 

 induction shocks may cause a tetaniform condition, if not a typical 



tetanus. The con- 

 traction of the 

 normally beating 

 heart is really a 

 simple contrac- 

 tion, and not a 

 tetanus. The 

 electrical changes 

 correspond to a 

 single contraction 

 (p.8o6);ar^iwhen 

 the nerve of a 

 nerve-muscle pre- 

 paration is laid 

 on the heart, the 

 muscle responds 

 Fig. 68. Refractory Period and Compensatory Pause {- o eac h beat bv a 



(Marey). A frog's heart was stimulated at a point corre- < . ., , 



spending to the nick in the horizontal line below each S111 :n > 



curve. In i and 2 there was no response; in 3 and 4 there 



was an extra contraction, succeeded by a compensatory 



pause< That the cardiac 



muscle itself, apart from the intrinsic nervous mechanism, shows 

 the phenomenon of ' refractory state ' has been shown in the 

 Limulus heart after extirpation of the ganglion (Carlson). 



Like ordinary skeletal muscle, the cardiac muscle is at first bene- 

 fited by contraction, perhaps by an ' augmenting ' action of fatigue- 

 products such as carbon dioxide (Lee), so that when the apex is 

 stimulated at regular intervals each contraction is somewhat 

 stronger than the preceding one. To this phenomenon the name of 

 the staircase or ' treppe ' has been given, from the appearance of the 

 tracings (p. 723). 



and not by tet- 

 anus (p. 201). 



SECTION V. THE NERVOUS REGULATION OF THE HEART 

 (EXTRINSIC NERVOUS MECHANISM OF THE HEART). 



While, as we have seen, the essential cause of the rhythmical beat 

 of the heart resides in the tissue of the heart itself, it is constantly 

 affected by impulses that reach it from the central nervous system. 

 These impulses are of two kinds, or, rather, produce two distinct 

 effects : inhibition, shown by a diminution in the rate or force of the 

 heart-beat, or in the ease with which the contraction is conducted 

 over the heart-wall; and augmentation, or increase in the rate or 



