CHAPTER XI 



THE NERVOUS SYSTEM 



TWENTY -FIVE years ago a new process was introduced for 

 colouring the elements which by their combination make up 

 the nervous system. With its aid anatomists discovered the 

 inadequacy of their conceptions of nerve-cells. It was already 

 known that a nerve-fibre that is to say, its essential part, its 

 core is a part of a cell, the body and other parts of which 

 are situate within the brain or spinal cord, or in one of their 

 dependents, a ganglion. But the new method showed the 

 nerve-cell as more elaborate in form than anything which had 

 been imagined hitherto ; and since the word " cell " was often 

 loosely used when the cell-body alone was referred to, it seemed 

 worth while to give the unit of structure a new name. The 

 term " neurone " was introduced to emphasize its functional 

 individuality. The nervous system is an association of 

 neurones. 



By the extremely simple expedient of placing a small block 

 of nerve-tissue in bichromate of potassium, and then trans- 

 ferring it to nitrate of silver, jet-black pictures of nerve-cells 

 are obtained showing with amazing completeness all the 

 details of contour of their bodies and all the intricacies of 

 branching of their limbs. The most surprising feature of the 

 process is the absence of confusion in its results. Dyes were 

 in use which stained one kind of cell better than another, or 

 picked out a particular part usually the nucleus of every 

 cell. If the chrome-silver process had acted in the same 

 way, a dense black preparation in which no details could be dis- 

 tinguished would have been the result. But instead of treating 

 all cells alike, the process blackens one cell here and another 

 there, leaving hundreds or thousands untouched. It shows 

 no preference for any particular kind of cell. In one section 



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