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have announced I would endeavor to solve, as regards the exci- 

 tant of the heart's action. 



2. Why does that excitant act rhythmically ? 



I believe it is easy to explain why the agent of excitation of 

 the heart* produces rhythmical contractions. I will suppose, first, 

 that the action is permanent. A part of the heart, ventricles, or 

 auricles, 'being dilated, receives an excitation in all its fibres si- 

 multaneously, and a contraction is produced. But, according to 

 the well-known law of Schwann, the exciting cause which is able 

 to give the impulse when the muscular fibres are long, is not able 

 to maintain the contraction when the fibres have been shortened. 

 Then, on account of this insufficiency of power of the cause of the 

 contraction, a dilatation ensues. We may present the fact in 

 other words, and say that the resistance to the contraction origi- 

 nating from the displacement of the constitutive matter of the 

 contractile tissues, increases in proportion to the shortening of 

 the fibres ; and that after the fibres have contracted under the 

 impulse of the exciting cause, although this cause continues to 

 act, a dilatation is produced by the force belonging to that re- 

 sistance, which is nothing but elasticity. If the cause of the con- 

 traction of the heart was a considerable one, then we should see 

 a permanent .contraction ; and it is so when we apply galvanism 

 the elasticity, then, is not powerful eno-ugh to produce dilatation. 

 On the contrary, with a weak exciting cause, like earbenic acid, 

 the result ought to be different. When that cause has more 

 power, as in asphyxia, the shortening of the fibres takes place 

 quicker, and is more considerable ; and even then it is not suffi- 

 cient to maintain contraction, the tendency to dilatatioa being 

 also increased. 



I ought to say, that the excitant cause of the contractions is 

 not always at the same degree of power. The small blood-vessels 

 and the capillaries being compressed diiring the muscular con- 

 tractions, there is a diminution of excitation during that time. 

 This should be sufficient to explain the alternate contractions 

 and dilatations. But such a diminution in the caliber ought to 

 be very little, if even it exists in certain organs, (the heart when 

 composed of cells, for instance.) 



* Whal I will .gay here for the heart, might be said for all the .contractile 

 tissues, presenting apparently spontaneous rhythmical contractions, as the 

 eiiia, for instance. 



