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 acthity 

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

 grey matti-r of the brain which possess this activity. 

 On the other hand, we are entirely unable even to 

 indicate ln>\v this consciousness comes into being. It 

 may be due to molecular processes in the nerve-cells 

 which 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. 1 



The excitements transmitted by the various sensory 

 Jibres do not all art 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-til MVS 

 which accomplish the various sensations differ in no 

 \vay from each other, we are forced to look in the 

 ner \e-cel Is for the reason of the difference in sensations. 

 .lust as we as-nmed th;it motor nerve-cells differ from 

 sensory, so we must further assume that among 

 sensory nerve-cells, ihe excitement of which always 

 elicits the conception of light, others again the excite- 



1 E. dii BOIE li> \ iii'ind bag entered further into this question in 



his addivs-; In the' a>srinb]y of nalurali-ts ;il I,rii>/.iir ( I < hi r die 

 firm:t'ii il<x ^'utiii-i'f/.'i'iiiiciin, Lcip/iir, is;-.'). Some of tbr \.<ui 

 natural philosophers srrin inclined to avoid the dillk-ully by a.-c'.rib- 

 in;.', as does Si-ln'i culiaucr, sensation and ronscimisno-; to all mole- 

 ruk-s, but tliis does not seem to nit- to be any real t, r ain. 



