282 ANALYSIS OF HEART-BEAT. [BOOK i. 



ventricle subsequently becomes distended the pressure in the chamber 

 is increased, and the piston and lever sink. In this way variations in 

 the volume of the ventricle may be recorded, without any great inter- 

 ference with the flow of blood or fluid through it. 



The heart of the frog, as we have just said, will continue to 

 beat for hours after removal from the body even after the cavities 

 have been cleared of blood, and indeed when they are almost empty 

 of all fluid. The beats thus carried out are in all important 

 respects identical with the beats executed by the heart in its 

 normal condition within the living body. Hence we may infer 

 that the beat of the heart is an automatic action : the muscular 

 contractions which constitute the beat are due to causes which 

 arise spontaneously in the heart itself. 



In the frog's heart, as in that of the mammal, 126, there is a 

 distinct sequence of events which is the same whether the heart be 

 removed from, or be still in its normal condition within, the body. 

 First comes the beat of the sinus venosus, preceded by a more or 

 less peristaltic contraction of the large veins leading into it, next 

 follows the sharp beat of the two auricles together, then comes the 

 longer beat of the ventricle, and lastly the cycle is completed by the 

 beat of the bulbus arteriosus, which does not, like the mammalian 

 aorta, simply recoil by elastic reaction after distension by the 

 ventricular stroke but carries out a distinct muscular contraction 

 passing in a wave from the ventricle outwards. 



When the heart in dying ceases to beat, the several movements 

 cease, as a rule, in an order the inverse of the above. Omitting 

 the bulbus arteriosus, which sometimes exhibits great rhythmical 

 power, we may say that first the ventricle fails, then the auricles 

 fail, and lastly the sinus venosus fails. 



The heart after it has ceased to beat spontaneously remains 

 for some time irritable, that is capable of executing a beat, or 

 a short series of beats, when stimulated either mechanically as 

 by touching it with a blunt needle .or electrically by an induction 

 shock or in other ways. The artificial beat so called forth may 

 be in its main features identical with the natural beat, all the 

 divisions of the heart taking part in the beat, and the sequence of 

 events being the same as in the natural beat. Thus when the 

 sinus is pricked the beat of the sinus may be followed by a beat 

 of the auricles and of the ventricle ; and even when the ventricle is 

 stimulated, the directly following beat of the ventricle may be 

 succeeded by a complete beat of the whole heart. 



Under certain circumstances however the division directly 

 stimulated is the only one to beat ; when the ventricle is pricked 

 for instance it alone beats, or when the sinus is pricked it alone 

 beats. The results of stimulation moreover may differ according 

 to the condition of the heart and according to the particular spot 

 to which the stimulus is applied. 



With an increasing loss of irritability, the response to stimula- 



