192 ANIMAL PHYSIOLOGY. 



the neck, where the origins of the nerves are further separ- 

 ated; and if it be made at a point midway between the 

 origins of successive nerves, not only is it painless, but 

 irritation of the lips of the wound produces no pain. From 

 this it is concluded that the proper fibres of the cord, those 

 not directly continuous with the nerves, are insensible to 

 irritation, and that the pain in the dorsal region is occasioned 

 by irritation to the fibres of the sensory nerve-roots as they 

 pass in to the grey matter, some of them passing upwards 

 and others downwards in their course to the posterior cornu, 

 so that there are divided fibres in connection with the grey 

 matter in both the lips of the wound. And not only are the 

 fibres proper to the cord insensible to direct irritation, but 

 so also is the brain. Slicing the brain away in experiments 

 on animals is painless, and disease of the substance of the 

 brain is painless, although inflammation of the membranes 

 covering it is acutely painful, the membranes having nerves 

 distributed in them. There is thus a very marked difference 

 in the irritability of the fibres of the central nervous system 

 and the peripheral nerves; it does not, however, follow that 

 the active or impressed condition is not of the same descrip- 

 tion in both. 



