CHAPTER XVI 

 THE CENTRAL NERVOUS SYSTEM 



IN other divisions of our subject we have been able to follow to a 

 greater or less extent the processes which take place in the organs 

 described. The chemistry and the physics of these processes have 

 bulked more largely in our pages than the anatomy and histology 

 of the tissues themselves. In dealing with the central nervous 

 system, we must adopt a method the very reverse of this. Its ana- 

 tomical arrangement is excessively intricate. The events which 

 take place in that tangle of fibre, cell, and fibril are, on the other 

 hand, almost unknown. So that in the description of the physiology 

 of the central nervous system we can as yet do little more than 

 trace the paths by which impulses may pass between one portion 

 of the system and another, and from the anatomical connections 

 deduce, with more or less probability, the nature of the physiological 

 nexus which its parts form with each other and the rest of the body. 

 And here it may be well to remark that, although for convenience 

 of treatment we have considered the general properties of nerves 

 in a separate chapter, there is not only no fundamental distinction 

 between the central nervous system and the outrunners which 

 connect it with the periphery, but obviously a central nervous 

 system would be meaningless and useless without afferent nerves 

 to carry information to it from the outside, and efferent nerves along 

 which its commands may be conducted to the peripheral organs. 



SECTION I. STRUCTURE OF THE CENTRAL NERVOUS SYSTEM 



HlSTOLOGICAL ELEMENTS. 



In unravelling the complex structure of the central nervous 

 system, we avail ourselves of information derived (i) from its gross 

 anatomy ; (2) from its microscopical anatomy ; (3) from its develop- 

 ment ; (4) from what we may call, although the term is open to the 

 criticism of cross-division, its physiological and pathological 

 anatomy. 



Certain tracts of white or grey matter are differentiated from each 

 other by the size of their fibres or ceils. For example, the postero- 

 median column of the spinal cord has small fibres, the direct cere- 



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