123 



I come now to the third question about the excitant of the 

 heart 



3. Does that excitant act together on the muscular fibres and 

 on the nerves of the heart, or does it act only on the muscular 

 fibres ? 



I believe at ought to act also on the nerves ; but I cannot prove 

 it otherwise tthan by saying, that all the agents of excitation that 

 we know to .act on the muscular fibres, are able to act on the 

 nerves. 



There are many things to be said besides the above facts and 

 reasoning, to prove the truth of the doctrine I propose. I will 

 expose some of them. 



The following question might be made : 



How is it that the heart is the only muscle containing striated 

 fibres, which presents normally rhythmical movements ? 



^ The answer to this question appears to be very simple. The 

 intensity of the stimuli, the degree of irritability, and the resist- 

 ance which a muscle has to overcome when it contracts, are three 

 elements which we ought not to lose sight of when we examine 

 the difference of contractions between two muscles. Suppose the 

 heart possessing the same degree of irritability as another mus- 

 cle : if the stimulus is the same, and the resistance the same also, 

 for the heart and for the other muscle, there will be the same 

 effects. But if the stimulus is more considerable in the heart 

 than in the other muscle, and if the resistance to be overcome is 

 less for the heart, then with the same degree of irritability in 

 both parts, and even with less irritability in the heart than in the 

 other muscle, we wiM see a movement in the heart, and not in 

 that other muscle. Now a simple examination of the vessels of 

 the heart, proves that they contain more blood, and consequently 

 more stimulus, than the other striated muscles. Besides, as the 

 heart is not inserted into heavy bones to be moved, it has less re- 

 sistance to overcome when it has not to circulate the blood, as 

 after death, or when it is out of the chest, than the muscles of 

 animal life. Some muscles in the face and the diaphragm, being 

 almost without an external resistance, when their contractions 

 do not go so far, it results that they are moved much more easily 

 after death, than the muscles of the limbs. In consequence of 

 these views, I believe that, although there is in die blood-vessels 



