318 ZOOLOGICAL PHILOSOPHY 



This electric fluid is incessantly penetrating into the blood, either by 

 respiration or some other method, and is there gradually modified 

 and animalised so as to acquire at length the properties of the nervous 

 fluid. Now we may probably regard the gangUa, spinal cord, and 

 especially the brain with its accessories, as the organs which secrete 

 this animal fluid. 



There is indeed reason to believe that the substance of the nerves, 

 which, on account of its albumino-gelatinous nature, is a better con- 

 ductor of the nervous fluid than any other substance of the body, 

 and far better than the aponeurotic membranes which invest the 

 nervous threads and cords, continually draws off the subtle fluid in 

 question from the minute arteries circulating in it. It is no doubt 

 these minute arteries and veins that give rise to the grey colour of the 

 external or cortical layer of the medullary substance. 



In this way, there is incessantly produced in animals with a nervous 

 system the subtle and invisible fluid which moves in the substance 

 of their nerves and in the medullary nuclei where these nerves terminate. 



This fluid works in the nerves by two kinds of movements in opposite 

 directions ; and the activities of the cerebral hemispheres suggest 

 that it carries out many and varied movements in them that are 

 beyond our powers of ascertaining. 



In the nerves which give rise to sensations, we know that the fluid 

 moves from the periphery of the body towards the centre, or rather 

 towards the nucleus that produces these sensations ; and since the 

 individuals which have a nervous system may also experience internal 

 impressions, the fluid in these cases moves in the nerves of the internal 

 regions, but still in the direction of the nucleus of sensations. 



In the nerves which produce muscular movement, on the contrary, 

 whether voluntary or involuntary, the nervous fluid moves from the 

 centre or common nucleus towards the parts which have to be moved. 



In both these cases where the nervous fluid moves in the nerves, 

 and also in the case of its various movements in the brain, the activity 

 of this fluid consumes a certain part of it, which is thereby dissipated 

 and lost. This loss necessitates a restoration, which is continually 

 being made by healthy blood. 



The following is a very important observation for understanding 

 the phenomena of organisation : 



Individuals, which use their nervous fluid only for the production 

 of muscular movement, make good their losses with interest, so that 

 their strength is increased ; since this muscular movement accelerates 

 the circulation and other organic movements, and the secretions 

 which make up for the used fluid are copious at the periods of rest. 



Individuals, on the other hand, who use their nervous fluid only in 



