278 PHYSIOLOGY OF MUSCLES AND NERVES. 



or as in sleep. The irritant can then elicit a reflex 

 action, though there is no consciousness of this. That 

 the origin of conscious conceptions is also an activity 

 of the nerves is certain, and it is the cells of the 

 grey matter of the brain which possess this activity. 

 On the other hand, we are entirely unable even to 

 indicate how this consciousness comes into being. It 

 may be due to molecular processes in the nerve-cells 

 w^hich result from the received excitement ; but mole- 

 cular processes are but movements of the molecules, 

 and though we can understand how such movements 

 cause other movements, we are entirely unaware how 

 these can be translated into consciousness.^ 



The excitements transmitted by the various sensory 

 fibres do not all act in the same way on the brain, and 

 the sensations to which they give rise differ. Accord- 

 ingly, we may distinguish the various sensations of 

 the various senses, and even within one and the same 

 sense various sub-species, as the colours in the sphere 

 of optical sensations, the various pitches in the sphere 

 of auditory sensations. But as all the nerve-fibres 

 which accomplish the various sensations differ in no 

 way from each other, we are forced to look in the 

 nerve-cells for the reason of the difference in sensations. 

 Just as we assumed that motor nerve-cells differ from 

 sensory, so we must further assume that among 

 sensory nerve-cells, the excitement of which always 

 elicits the conception of light, others again the excite- 



• E. du Bois-Reymond has entered further into this question in 

 his address to the assembly of naturalists at Leipzig ( Uchcr die 

 Grenzen des Kaiurei'liennens, Leipzig, 1872). Some of the younger 

 natural philosophers seem inclined to avoid the difficulty by ascrib- 

 ing, as does Schopenhauer, sensation and consciousness to all mole- 

 cules, but this does not seem to me to be any real gaift. 



