264 Dr. A. P. W. Philip on the 



is influenced by all agents applied to any considerable part of 

 the brain or spinal marrow, while the muscles of voluntary mo- 

 tion are only influenced by the more powerful agents applied to 

 certain small parts of them. 



These facts being ascertained, the other differences observed 

 in the effects of agents applied to the brain and spiral marrow 

 on the heart and muscles of voluntary motion are easily 

 explained. 



Irregular action of a muscle arises from stimuli acting par- 

 tially or at intervals on its nerves, or on the part of the brain 

 or spinal marrow from which its nerves arise ; but very partial 

 action of an agent on these organs, we have just seen, is 

 incapable of exciting the heart; and while the agent is applied 

 to any part of them, as all their parts seem equally to influence 

 the heart, it cannot act upon it interruptedly, as an instrument 

 does on the muscles of voluntary motion, when it is moved from 

 place to place in the brain*. 



The heart feels the eifect of the agent within certain limits, as 

 long as it is applied to the brain and spiral marrow, while the 

 muscles of voluntary motion chiefly feel its first impression; 

 because they feel only the effects of powerful agents, applied 

 to certain small parts of these organs, which, being strongly 

 impressed, soon lose their excitability ; while the heart feels 

 the sum of all, even slight impressions, made on every part of 

 themf. 



It also appears, from what has been said, why those who 

 Jiave endeavoured to influence the heart by stimulating the small 

 parts of the brain from which its nerves seem chiefly to originate, 



* It is true, that although the heart is only influenced by ageuts applied 

 to a large portion of the brain, we may conceive tliein so applied as to pro- 

 duce irregular action in it, and we find that certain irritations of the 

 nervous system have tliis effect. Suddenly crushing part of the brain or 

 spinal marrow renders the action of tlie heart irregular. But it is evident 

 tliat the heart, not being subject to agents whose action is confined to a 

 small portion of these organs, and being equally affected through all parts of 

 them, must render it much less subject to irregular action, and readily ac- 

 counts for this not having been observed in the experiments just referred to. 



t Exper. Jnq., Expcr. 34. 



