THE NEKVOUS SYSTEM. 



end of the proximal portion, i. e., of the portion still connected with 

 the nervous centre, sensation is perceived, or a reflex action ensues; but 

 when the end of the distal portion of the divided nerve is irritated, no 

 effect appears. When an impression is made upon any part of the course 

 of a sensory nerve, the mind may perceive it as if it were made not only 

 upon the point to which the stimulus is applied, 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 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 ulna 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 pressure 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. 



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

 deprived of sensibility by compression or division of the nerves supply- 

 ing them, irritation of the portion of the nerve connected with the brain 

 still excites sensations which are felt as if derived from the parts to which 

 the peripheral extremities of the nerve-fibres are distributed. Thus, 

 there are cases of paralysis in which the limbs are totally insensible to 

 external stimuli, yet are the seat of most violent pain, resulting appar- 

 ently from irritation of the sound part of the trunk of the nerve still in 

 connection with the brain, or from irritation of those parts of the ner- 

 vous centre from which the sensory nerve or nerves which supply the 

 paralyzed limbs originate. An illustration of the same law is also afforded 

 by the cases in which division of a nerve for the cure of neuralgic pain 

 is found useless, and in which the pain continues or returns, though 

 portions of the nerves are removed. In such cases, the disease is probably 

 seated nearer the nervous centre than the part at which the division of 

 the nerve is made, or it may be in the nervous centre itself. In the same 

 way may be explained the fact, than when part of a limb has been re- 

 moved by amputation, the remaining portions of the nerves may give rise 

 to sensations which the mind refers to the lost part. When the stump 

 is healed, the sensations which we are accustomed to have in a sound limb 

 are still felt; and tingling and pains are referred to the parts that are 

 lost, or to particular portions of them, as to single toes, to the sole of the 

 foot, to the dorsum of the foot, etc. 



It must not be assumed, as it often has been, that the mind has no- 

 power of discriminating the very point in the length of any nerve- 

 fibre to which an irritation is applied. Even in the instances referred 

 to, the mind perceives the pressure of a nerve at the point of pressure, 

 as well as in the seeming sensations derived from the extremities of the 

 fibres: and in stumps, pain is felt in the stump, as well as, seemingly, 

 in the parts removed. It is not quite certain whether these sensations 

 are due to conduction" through the nerve-fibres which are on their 



