5G8 THE HUMAN BODY. 



The matter cannot, however, be at present decided. The? 

 skin seems indubitably to contain many nerve-fibres which 

 terminate in free axis cylinders without end organs; and 

 these may well be true pain-fibres, fitted to respond to- 

 stronger stimuli than those which excite the tactile and 

 thermic end organs. Certain pathological and experi- 

 mental phenomena tend also to prove that the brain-centres 

 concerned in the production of tactile and painful sensa- 

 tions are different. Persons sometimes lose pain sensations 

 and keep tactile; a gentle touch is felt as well as usual, but 

 a powerful pinch causes no pain: in one stage of ether and 

 chloroform narcosis the same thing is observed; the sur- 

 geon's hand and knife are felt where they touch the skin, 

 but cutting deeper into the tissues produces no pain. In 

 animals a similar state of things may be produced by divid- 

 ing the gray matter of the cord, leaving the posterior white 

 columns intact; while, if the latter be divided and the gray 

 substance left uninjured, there is increased sensitiveness to- 

 pain, and probably touch proper is lost, though that is im- 

 possible to say with certainty. Such experiments make it. 

 pretty certain that when sensory afferent impulses reach 

 the cord at any level and there enter its gray matter with 

 the posterior root-fibres, they travel on in different tracks 

 to conscious centres; the tactile coming soon out of the- 

 gray network and coursing on in a readily conducting white 

 fibre, while the painful first travel on farther in the gray sub- 

 stance. It is still uncertain if both impulses reach the cord 

 in the same fibres. The gray network conducts nerve im- 

 pulses, but not easily; they tend soon to be blocked in it, 

 A feeble (tactile) impulse reaching it by an afferent fibre- 

 might only spread a short way and thnn pass out into 

 a single good conducting fibre in a white column, and 

 proceed to the brain; while a stronger (painful) impulse 

 would radiate farther in the gray matter, and perhaps 

 break out of it by many fibres leading to the brain through 

 the white columns, and so give rise to an inco-ordinate and 

 ill localized sensation. That pains are badly localized, and 

 worse the more intense they are, is a well-known fact,, 

 which would thus receive an explanation (see p. 579), 



