380 FUNCTIONS OF THE NERVOUS SYSTEM. [cH. i. 



SECTION VIII. THE FUNCTIONS OF THE NERVOUS SYSTEM ARE 



EXPLAINED BY THE VIS NERVOSA. 



At length we abandon the Cartesian method of philoso- 

 phising in this part of animal physics also, and adopt the 

 Newtonian, being persuaded that the way to truth through 

 hypotheses and conjectures is tedious and altogether uncertain, 

 but far more certain, more excellent, and shorter, through the 

 inductive method. Newton designated the mysterious cause 

 of physical attraction by the term vis attractiva, observed and 

 arranged its effects, and discovered the laws of motion ; and 

 thus it is necessary to act with reference to the functions of 

 the nervous system : we will term the cause latent in the pulp 

 of the nerves, producing its effects, and not as yet ascertained, 

 the vis nervosa : we will arrange its observed effects, which are 

 the functions of the nervous system, and discover its laws ; and 

 thus we shall be able to found a true and useful doctrine, which 

 will undoubtedly afford a new light, and more elegant character 

 to medical art. The illustrious Haller has already used the 

 phrase vis nervosa, in designating the agent which the nerves 

 employ in exciting muscular contractions ; but the celebrated 

 and ingenious J. A. Unzer has thrown the greatest light on 

 the subject -^ for although he continues the use of the term 

 animal spirits, that he may the more conveniently and intel- 

 ligibly express himself, yet, as he himself observes, his whole 

 system is complete without them.^ 



• Vid. Grundrisz eines Lehrgebaudes von der Sinnlichkeit der thierischen Korper, 

 &c. 1768. Also, Erste Griinde einer Physiologic der eigentlichen thierischen Natur 

 thierischer Korper, 1771. 



^ Prochaska thus explains in what sense he uses the term vis nervosa, in the 

 " Address to the Reader," prefixed to the edition of this dissertation, pubUshed in 

 his ' Opera Minora,' part 2. (Vienna, 1800.) "I had already (in 1780) published 

 this dissertation, in the third fasciculus of my * Adnotationes AcademiccB ;^ at which 

 time many philosophers, and the distinguished Tissot himself, still used the hy- 

 pothesis of a nervous fluid, to explain the functions of the nervous system in accord- 

 ance with the opinion of Boerhaave. Convinced of the insufficiency of this hypothesis, I 

 resolved to use the inductive method in this dissertation, and explain those functions 

 by facts only ; using the term * vis nervosa' to designate that agent (as yet unknown) 

 by which the nervous system is rendered fit for the performance of its functions, 

 and which I have used more extensively in my pubUc lectures, and in my institutes 

 of human physiology (' Lehrsatze aus der Physiologic des Menschen,' 1797)." — Ed. 



