THE NERVOUS FLUID 315 



I may be mistaken, but I confess that I am of quite a different 

 opinion ; I am firmly convinced that these same substances play an 

 important part in most of the physical facts that we observe, and 

 especially in the organic phenomena presented by living bodies ; and 

 hence that their investigation is of the greatest importance for the 

 progress of knowledge on these subjects. 



Thus, although we cannot know directly all the subtle substances 

 existing in nature, yet if we were to abandon all enquiry with regard to 

 some of them, we should in my opinion be rejecting the only clue that 

 can lead to a knowledge of natural laws ; we should be giving up the 

 hope of real progress in our knowledge of living bodies, as also of the 

 causes of the phenomena that we observe in their functions ; we should 

 at the same time be relinquishing the only path that can lead to the 

 perfection of our physical and chemical theories. 



It will soon be clear that these remarks are not irrelevant to my 

 purpose, and indeed that they are entirely applicable to that nervous 

 fluid about which we so greatly desire information. 



Since our observations are now too advanced to permit of any real 

 doubt as to the existence of a subtle fluid which circulates and moves 

 about in the substance of the nerves, let us see how far we are led, on so 

 delicate and difficult a subject, by the actual state of knowledge. 



But before speaking of the nervous fluid, it is very important to 

 estabhsh the following proposition : 



All the visible fluids contained in an animal's body, such as the 

 blood or substitute for the blood, the lymph, secreted fluids, etc., move 

 too slowly in the canals or parts which contain them, to be capable of 

 conveying with sufficient rapidity the movement or cause of move- 

 ment which produces actions in animals ; for these actions are carried 

 out in many animals with an amazing rapidity and vivacity, and the 

 animals can interrupt them, start them again, and vary them with all 

 possible degrees of irregularity. The slightest reflection should suffice 

 to convince us that it is absolutely impossible that fluids so gross as 

 those just mentioned, and whose movements are usually so regular, 

 can be the cause of the various actions of animals. Yet evervthing 

 that passes in them results from relations between their contained 

 fluids, or such as penetrate into them, and their containing parts, or 

 the organs affected by these contained fluids. 



Assuredly it can only be a fluid, moving almost with the swiftness 

 of light, that could work such effects as those I have named ; now we 

 have some knowledge of fluids which possess this faculty. 



All action is the product of movement, and the nerves certainly 

 act by a movement of some sort ; the opinion of those who regarded 

 the nerves as vibrating cords has been discussed and effectually refuted 



