Il6 IRRITABILITY OF THE AUTOMATIC MOTOR CENTERS. 



7. Each contraction of the heart is followed by a short period during 

 which the heart is less susceptible to subsequent stimuli (Marey's re- 

 fractory period) and the conducting-power of the muscle-substance is 

 reduced. 



8. Stimulation of the heart-centers, apparently reflex, takes place 

 on the inner surface of the heart. Feeble stimuli from this surface are 

 more effective in accelerating and exciting the action of the heart than 

 stimuli from the external surface of the heart. Stronger stimuli, which 

 cause arrest of the heart, also act more readily from the internal than 

 from the external surface of the heart; under such conditions also the 

 ventricular portion is always first to be paralyzed. 



9. Portions of the heart that are devoid of ganglia are incapable of 

 independent movement unless a stimulus be applied ; they contract only 

 once to a single direct stimulus, or they may beat rhythmically if the 

 stimuli are applied continuously. Such a stimulus may be provided by 

 the continuous pressure of fluid within the cavities of the heart or by 

 means of chemical agents brought in contact with the heart. 



10. The pulsations of stimulated portions of the heart devoid of 

 ganglia indicate that the ganglia are not absolutely necessary for the 

 production of rhythmic contractions ; but the ganglia are more irritable 

 than the muscle itself. They control also the regular alternating action 

 of the various portions of the heart, so that normal cardiac action must 

 be regarded as under the control of the ganglia. 



11. If the heart be cut in such a way that the individual pieces 

 remain in communication, the regular contractions beginning in the 

 auricles and propagated in peristaltic or undulating movements to 

 the ventricles persist for some time. When, however, the heart is 

 completely divided into two pieces, auricle and ventricle, the movements 

 of both continue separately naturally, no longer in orderly succession, 

 but quite independently. 



The principal experiments on which the foregoing propositions are based are 

 as follows: 



Experimental Division and Ligation of the Heart. These experiments have 

 been performed chiefly on frogs' hearts. Ligation differs from division in the fact 

 that the physiological connection is destroyed by drawing a ligature tightly around 

 the parts and loosening it again, while the anatomical continuity of the heart -wall 

 and the integrity of the cavities of the heart are maintained. 



i. Stannius' Experiment. After separation in a frog's heart of the sinus of 

 the venae cavae from the auricle, either by incision or by constriction, the heart 

 is arrested in diastole, while the sinus continues to beat independently. If the 

 heart be again divided at the auriculo- ventricular junction, the ventricle, as a 

 rule, begins at once to beat again, while the auricles continue in diastolic arrest. 

 In accordance with the position of the second line of division the auricles may 

 continue to beat in association with the ventricles, or the auricles alone may 

 contract, while the ventricles remain at rest. 



The experiment has been interpreted in the following manner: The sinus of 

 the venae cavae contains Remak's ganglion, which is remarkable for its extreme 

 irritability, while Bidder's ganglion, which is situated at the auriculo-ventricular 

 junction, possesses a lesser degree of irritability. In the normal heart the latter 

 receives its motor impulses from the former. W nen the sinus of the venae cavse 

 is severed, the stimulating Remak's ganglion is without any influence on the heart. 

 The latter becomes arrested for two reasons: because Bidder's ganglion by itself 

 does not possess .sufficient power to set the heart in motion, and because the 

 division stimulates the inhibitory nerves of the heart (vagus), which are situated 

 at this point. Pulsation can, however, be induced in a heart that has been ar- 

 rested in this way by irritation of Bidder's ganglion, as by gently pricking the 

 auriculo-ventricular junction, or by the passage of a moderately strong constant 



