THE NERVOUS SYSTEM 295 



advance in our knowledge of the finer structure of the nervous 

 system which has marked the period during which they have 

 been employed. 



The chrome- silver process is the more useful for the central 

 nervous system. Methylene blue gives better results with 

 tissues containing minute nerve-cells and the branches of 

 nerves. The latter method has revealed such a profusion of 

 nerve-twigs as would never have been suspected but for its use. 

 Consider, for example, the lining epithelium of the lungs (p. 168). 

 Every one of its flattened cells has its own nerve twig or twigs. 

 They lie between the cells. They give branchlets which enter 

 them. A similar statement might be made regarding the 

 richness of the nerve-supply of the muscle-fibres of the ali- 

 mentary canal, or of the cells of glands, and possibly of other 

 tissues. Each fresh success achieved in the application of the 

 method makes a further revelation of the abundance in which 

 nerves are distributed, increasing our sense of the dependence 

 of all vital processes upon nervous control, and our appreciation 

 of the unifying and integrating importance of the nervous 

 system. 



The term " neurone " is used by certain writers with a view 

 to emphasizing their belief, not in the functional individuality 

 alone of the unit of structure, but also in its anatomical 

 isolation. The peculiarity of the methods of coloration which 

 we have described lies, as already pointed out, in their selecting 

 the cells which happen to be in a particular nutritive condition, 

 and ignoring their neighbours. Hence pictures of separate 

 and discrete units are obtained. This proves the nutritive 

 autonomy of the cells, but it does not necessarily follow that A 

 is not structurally connected with B, and B with C. Impulses 

 are passed along the chain from A to C. Functionally, there- 

 fore, they are linked together ; but until the question as to the 

 way in which contact is established is settled, it is as well to 

 think of the neurones as anatomically discrete. 



It would be impossible in this book to describe all the 

 varieties of neurone, for nothing is so characteristic of these 

 elements as their enormous range both in size and form. It 

 may be truly described as having no limits. Each of the two 

 electric organs of Malapterurus is governed by a single neurone. 

 Its cell-body is a fifth of a millimetre or more in diameter 



