THE NERVOUS SYSTEM 219 



or the nerve-cell, and the nerve-fibre, so called from their 

 microscopic appearance. Now these two tissues are not com- 

 monly mingled together, but either form separate organs or 

 distinct parts of the same organs. This leads us to the con- 

 clusion that their respective 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 instruments 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 outlying 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 ner- 

 vous 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 impression, and of being by it excited into activity. A ray 

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

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

 extremity within the brain, communicating with a nerve-cell, 

 the latter in its turn is excited, and the sensation of sight is 

 produced. (Read Note 4.) 



4. The Relations of the Brain and Sympathetic Nerve. " Buried in 

 the hidden recesses of the body, between the spinal column and the great 

 organs of nutrition, there is a double row of small knots of nervous sub- 

 stance, bound together by a series of nerves running from one to another, 

 in succession, from the neck to the base of the column. The whole 

 appears like a long, fine cord, with knots at various distances a collec- 



20. What comparison is made between the brain and the nation's capital? The vital 

 property, excitability ? What example is given ? 



