890 THE NERVOUS SYSTEM IN GENERAL, [ch. ii. 



of the medullary pulp of the nervous system is almost inor- 

 gauic j but much is still wanting, to enable us to understand 

 its admirable functions. We may assert, however, without 

 fear of contradiction, that an invisible element enters into its 

 composition, and that this constitutes the producing cause of 

 all the functions of the nervous system. Since this is as 

 mysterious and unknown as the vis attractiva of matter, it 

 seems to me that it may be termed with propriety the vis 

 nervosa. I leave the inquiry, as to its nature, to the very 

 sagacious and ingenious men already engaged in philosophical 

 experiments. I shall only attempt to determine some of its 

 general properties, before I enter upon the special functions 

 of the nervous system.^ 



1. A stimulus is necessary to the action of the vis nervosa. — 

 i. Although this vis nervosa is a property inherent in the me- 

 dullary pulp, it is not the chief and sole cause that excites the 

 actions of the nervous system, but is ever latent, and exists as 

 a predisposing cause, until another exciting cause, which we 

 term stimulus, is brought to bear. As the spark is latent in 

 the steel or flint, and is not elicited, unless there be friction 

 between the flint and steel, so the vis nervosa is latent, nor 

 excites action of the nervous system until excited by an applied 

 stimulus, which continuing to act, it continues to act, or if 

 removed, it ceases to act, or if re-applied, it acts again. 



ii. This stimulus is divided into stimulus of the body and of 

 the mind, — This stimulus is double : either it is some fluid or 

 solid body applied internally or externally to the nervous 

 system, and termed corporeal, or mechanical stimulus ; or else 

 is a mental stimulus present in a portion of the nervous system, 

 and by means of this portion controls the rest of the nervous 

 system, and the rest of the body, as far as it is allowed. 

 Whether this mental stimulus takes place through a system of 

 occasional causes, or pre-established harmonies [harmonia prae- 

 stabilita] , or, as assumed by many, by a physical influx, matters 

 little to our object; it is sufficient for us that the soul can 

 excite the nervous system to the performance of certain actions, 

 and this power we call a mental stimulus. 



iii. The relations of the actions of the nervous system to the 



1 I have conjectured, however, that there is an analogy between the vis nervosa 

 and electricity, in my Inst. Physiol. $ 206. 



