THE VASCULAR MECHANISM. 211 



the cardiac muscle possesses an inherent power of rhythmic contraction, 

 which is most marked in the sinus and least so in the ventricle, and 

 that this rhythmic power can continue absolutely independently of 

 either the central nervous system or the nerve cells in the heart, 

 although, as will be seen later, it can be influenced by impulses passing 

 along the nerves to the heart. This view is known as the myogenic 

 theory of the heart beat. The myogenic theory has been accepted for 

 the following reasons. 



(1) In the first place, it was shown by Gaskell that a strip of the 

 ventricle of the tortoise, if kept stretched and moist, can be made to 

 beat rhythmically, and will then continue to beat without any external 

 stimulus, although subsequent histological examination of the strip 

 shows that it contains no nerve cells. In the same way the apical half 

 of the frog's ventricle, which is free from nerve cells, although it will 

 not beat spontaneously, can be made to contract rhythmically if it is 

 fed with fluid through a cannula at a pressure sufficient to put tension 

 on the muscle fibres. 



(2) Secondly, it is possible in the frog's heart to remove almost 

 completely the ganglia of Bidder and Remak without disturbing the 

 cardiac rhythm in any way. 



(3) Thirdly, if, in a normally beating heart, successive single stimuli 

 are applied to the ventricle more frequently than the rate at which the 

 heart is beating, the rhythm of the heart can be reversed, so that the 

 beat starts in the ventricle and passes to the auricle, and then to the 

 sinus. Such a reversal of rhythm is quite incompatible with the 

 neurogenic theory of the heart beat, since it contradicts the general 

 law (law of forward direction) that nervous impulses can pass through 

 a synapse only in one direction. 



It may be concluded, therefore, (1) that the rhythmic contraction of 

 the heart is myogenic in origin, and (2) that, although all parts of the 

 heart possess some rhythmic power, the beat normally always starts in 

 the sinus, in which this power is most fully developed. The sinus sets 

 the pace of the heart, and the ventricle responds to the stimuli reaching 

 it, doing its work under the control of the sinus. 



The Propagation of the Beat. In the frog the muscular tissue of 

 the whole heart is continuous, but the power of the tissue uniting the 

 sinus and the auricles and the auricles and ventricle to conduct impulses 

 is less than that of the rest of the heart. The impulses starting from 

 the sinus are slightly delayed, therefore, in their passage to the 

 auricles, and again in their passage from auricles to ventricle, so that 

 there is a distinct pause between the contractions of the sinus and 

 auricles, and auricles and ventricle respectively. 



