FUNCTIONS OF NERVE-FIBRES. 481 



end of the distal portion of the divided nerve is irritated, 

 no effect appears. The absence of effect in the latter place 

 is, perhaps, not to be ascribed to the distal portion of the 

 nerve being completely cut off from connection with the 

 nervous centre, for it may contain fibres which, after 

 reaching their destination, return through loops back to a 

 nervous centre ; rather, it may be believed, that the sensi- 

 tive fibres cannot convey impressions in any direction ex- 

 cept towards the nervous centres. Concerning these things, 

 however, our knowledge is very imperfect. 



When an impression is made upon any part of the course 

 of a sensitive nerve, the mind may perceive it as if it were 

 made not only upon the point to which the stimulus is ap- 

 plied, but also upon all the points in which the fibres of the 

 irritated nerve are distributed : in other words, the effect 

 is the same as if the irritation were applied to the parts 

 supplied by the branches of the nerve. When the whole 

 trunk of the nerve is irritated, the sensation is felt at all 

 the parts which receive branches from it ; but when only 

 individual portions of the trunk are irritated, the sensation 

 is perceived at those parts only which are supplied by the 

 several portions. Thus, if we compress the ulnar nerve 

 where it lies at the inner side of the elbow-joint, behind 

 the internal condyle, we have the sensation of "pins and 

 needles," or of a shock, in the parts to which its fibres are 

 distributed, namely, in the palm and back of the hand, 

 and in the fifth and ulnar half of the fourth finger. When 

 stronger pressure is made, the sensations are felt in the 

 fore-arm, also ; and if the mode and direction of the pres- 

 sure be varied, the sensation is felt by turns in the fourth 

 finger, in the fifth, and in the palm of the hand, or in the 

 back of the hand, according as different fibres or fasciculi 

 of fibres are more pressed upon than others. 



It is in accordance with this law, that when parts are 

 deprived of sensibility by compression or division of the 

 nerve supplying them, irritation of the portion of the nerve 



ii 



