General Principles of Physiology. 265 



have failed ; and why the heart may be influenced through this 

 organ and the spinal marrow, after their power is too far reduced 

 to excite the muscles of voluntary motion*. As these only 

 obey agents applied to one part, if the change there be not suffi- 

 ciently great to produce the effect, it can be assisted by no other. 

 Thus T have found by experiment, that a blow which affects the 

 brain generally, without materially injuring it, produces com- 

 paratively little effect on the muscles of voluntary motion, but 

 it produces a great effect on the heart, because it feels the sum 

 of all the impressions. The nervous system, therefore, may be 

 so far exhausted as not to admit of the vivid impressions neces- 

 sary to excite the muscles of voluntary motion, and yet capable 

 of those which influence the heart. 



The heart, however, is not the only organ which receives 

 nervous influence from every part of the brain and spinal 

 marrow. The power of the blood vessels, we have seen, may 

 be destroyed by the sudden destruction of either of these organs, 

 and it also appears, from direct experimentsf , that they may be 

 influenced, even in the extremities, by agents applied even to 

 the upper surface of the brain. The alimentary canal may 

 also be influenced through both the brain and spinal marrow. 

 From the extreme irregularity of the motions of this canal, 

 we cannot ascertain whether it is subject to the influence of 

 the different parts of the brain and spinal marrow, in the way 

 in which this was done respecting the heart and blood vessels. 

 I therefore endeavoured to ascertain this point by experiments of 

 a different kind ; from which it appears, that on withdrawing 

 a great part of the influence of either the brain or spinal marrow, 

 the stomach is affected in a way which I shall soon have occa- 

 sion to consider more particularly]^. 



We may easily conceive why the muscles of voluntary motion 

 are excited when those parts of the brain or spinal marrow from 

 which they receive their nerves are stimulated ; but it seems 

 at first view more difficult to account for the heart and other 



* Exper. Inq.y Exper. 38. 

 t Erper. Iri'i., Exper. 2G, 27. 

 : lb.. Chap. VH. sect. II. 



