PHYSIOLOGY, 17 



The condition fitting the medullary substance for having mo- 

 tion propagated in it, we suppose to be the presence of a certain 

 fluid, which we therefore name the nervous fluid, without mean- 

 ing, however, at present to determine any thing with regard to its 

 source, nature, or manner of acting. " It has been questioned . 

 among physicians whether or not this communication of motion 

 along the nerves does take place ; but there are few of opinion 

 that it does not, and it is almost universally admitted that there 

 is a continuation of motion from the origin to the extremities, 

 and in the contrary direction. You probably know, that it is 

 disputed what is the foundation of this communication of mo- 

 tion, what it is that fits the nervous system for it. Some sup- 

 pose that the nerve is an elastic chord, and that the communica- 

 tion is by a tremor, as in other elastic bodies ; while others believe 

 that there is a nervous fluid moving from the brain to the ex- 

 tremities, and that by this fluid a motion may be propagated 

 from the extremities of the nerves to the brain : a third sup- 

 position is made by Sir Isaac Newton, who imagines that the 

 nerves may be considered as solid capillamenta, and that a 

 fluid adhering to these is fitted to communicate the motions 

 from one part to another. 



" Now to avoid determining any thing with regard to these 

 opinions, I have used the term of nervous power ; but as this 

 is a little ambiguous, I choose to express it by nervous fluid ; 

 not that I suppose, with Dr. Boerhaave, that the brain is an ex- 

 cretory, and that a fluid is secreted from it : I mean nothing 

 more than that there is a condition of the nerves which fits them 

 for the communication of motion. But I defer the consideration 

 of these opinions for the present, and perhaps ad Grcecas ca- 

 lendas ; but nothing shall be rested upon the nervous fluid, it 

 shall be considered merely as a power fitted for communicating 

 motions." 



XXXI. In the living man there is an immaterial thinking 

 substance or MIND constantly present ; and every phenomenon 

 of thinking is to be considered as an affection or faculty of the 

 mind alone. But this immaterial and thinking part of man is 

 so connected with the material and corporeal part of him, and 

 particularly with the nervous system, that motions excited in 

 this give occasion to thought ; and thought, however occasioned^ 

 VOL. i. B 



