THE XERVOUS SYSTEM. 159 



19. The Properties of Nervous Tissue. We have 

 seen that in all parts of this system, there are only two 

 forms of nervous tissue; namely,. the gray substance and 

 the white substance, so called from their difference of color 

 as seen by the naked eye; or the nerve-cell, and the nerve- 

 fibre, so called from their microscopic appearance. Now 

 these two tissues are not commonly mingled together, but 

 either form separate organs, or distinct parts of the same 

 organs. This leads us to the conclusion that their respec- 

 tive uses are distinct. And this proves to be the simple 

 fact; wherever we find the gray substance, we must look 

 upon it as performing an active part in the system, that is,, 

 it originates nervous impulses ; the white matter, on the 

 contrary, is a passive agent, and serves merely as a con- 

 ductor of nervous influences. Accordingly, the nervous 

 centres, composed so largely of the gray cells, are the great 

 centres of power, and the white fibres are simply the in- 

 struments by which the former communicate with the near 

 and distant regions of the body under their control. 



20. We may compare the brain, then, to the capital, or 

 seat of government, while the various ganglia, including 

 the gray matter of the cord, like so many subordinate 

 official posts, are invested with authority over the outly- 

 ing provinces; and the nerves, with the white matter of 

 the cord, are the highways over which messages go and 

 return between these provinces and the local or central 

 governments. But both forms of nervous tissue possess 

 the same vital property, called excitability ; by which term 

 is meant, that when a nerve-cell or fibre is stimulated by 

 some external agent, it is capable of receiving an impres- 

 sion and of being by it excited into activity. A ray of 

 light, for example, falling upon one extremity of a fibre 

 ill the eye, excites it throughout its whole length ; and its 



1 9. Properties of nervous tissue ? Office of the gray substance ? Of the white ? 

 "he nervous centres? White fibres ? 



20. What comparison is made between the brain and the nation's capitol ? 

 The vital property, excitability ? What example is given ? 



