to the Agancy of the Nerves. 109 



When a change is produced in any of the functions of the ani- 

 mal economy, by means of a change effected in the nervous sys- 

 tem, two explanations of the fact immediately present them- 

 selves. 



It may either be supposed, that the ordinary exercise of the 

 function affected is dependant on some particular condition of the 

 nervous system, and, of course suffers a change corresponding to 

 that which the nervous system undergoes ; or, without any 

 such habitual dependence of the function affected on the ner- 

 vous system, it may be supposed liable to change from changes 

 in that system. The former supposition is perhaps the more ob- 

 vious of the two ; but there is nothing in the nature of things to 

 prevent the latter from being the true one, it being just as pro- 

 bable (i priori, that muscular actions, or secretions, taking place 

 independently of nerves, should be liable to modification, from 

 changes produced in nerve, as that that power, so totally different 

 from any possessed by nerves themselves, should be dependant 

 on any influence which nerves can supply. 



Unless, therefore, we have an experimentum crucis, to deter- 

 mine which of these explanations, in any particular case, is the 

 true one, we are not entitled, from seeing an alteration, or 

 even the cessation of any function of the animal economy, 

 brought about by changes in the nervous system, to infer the 

 dependance of that function, in ordinary circumstances, on any 

 thing derived from the nerves. 



Thus, Le Gallois was not entitled to conclude, from seeing 

 the action of the heart stopped in his experiments, by sudden 

 destruction^ of the spinal marrow, tliat the ordinary actions of 

 the heart are dependant on, or its life derived from, the spinal 

 marrow; because, without making that supposition, we can 



proving that the heart may be directly simulated by impressions made on 

 llic nervous system. But although the term stimulus was very naturally 

 applied by him to the substances which quickened the action of the heart 

 when applied to tiie brain or spinal marrow, yet various considerations 

 mi(;ht be stated to shew, that the effect of these substances is more correctly 

 expressed by saying, not that tliey excited contractions of (lie muscular 

 fibres of the heart, but that they increased the tendency of the muscular 

 fibres of the heart, to contract from the stimulus of (he Ijlood. 



