92 PHYSIOLOGY. 



whether it has been frequently or accurately examined I will 

 not determine. We have been frequently exercised in observ- 

 ing the motions of the heart, after it was cut out from the body 

 and freed from all connexion with the animal power. If you 

 will allow the heart to remain without any impression, and pre- 

 serve it in a pretty uniform temperature, it is found that its little 

 palpitations will continue for perhaps an hour. Now, supposing 

 that the heart thus left to itself unstimulated, will continue to 

 move so long, but, as it has its little intermissions, becoming more 

 languid ; if, in these intervals, you prick it with a needle, it will 

 renew its vibrations with more vigour ; but if you do employ 

 these stimuli, and frequently excite to a greater degree its con- 

 tractions, it is alleged that it will finish sooner entirely than it 

 would have done ; so that if you keep frequently pricking at 

 the heart with a needle, it will lose its irritability, and cease to 

 move perhaps in half an hour. Here we suppose that it is ex- 

 cited with unusual force, and that gives a kind of lassitude or 

 debility, analogous to what happens in voluntary motions ; and 

 if so, the circumstance belongs to the muscular fibre itself. 

 But further I cannot go ; and, upon the contrary, there are 

 many circumstances that lead us to answer the above question 

 in the affirmative, that the actions which depend upon the 

 animal power only are liable to the debility which arises up- 

 on their repetition. What led to the question is a passage in 

 Dr. Haller, which I wish he had proved a little further, (Primce 

 Linece, 119.) ' Sed quaeri potest, Sec. 1 ' Why, when the 

 most part of the muscles of the body are exposed to lassitude 

 and debility, has the heart this singular property, that for many 

 thousand times in a day, for all the days of so many years of 

 life, it goes on without discovering any marks of lassitude and 

 debility?' It is perhaps of some use to consider this. I think 

 it may be explained in this way, that the heart has its action 

 excited without that occasional concurrence of the animal power 

 which is necessary in the voluntary motions, that it depends 

 upon some constant energy derived from the brain, which I 

 have demonstrated. But it does not require that concurrence 

 of animal power, which is necessary to voluntary motions, but 

 not required in such as are never spontaneously omitted and usu- 

 ally carried on without the power of the will. The action of 



