380 THE NERVOUS SYSTEM. 



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 

 forearm 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. 



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

 prived of sensibility by compression or division of the nerve 

 supplying them, irritation of the portion of the nerve connected 

 with the brain still excites sensations which are felt as if de- 

 rived 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 apparently 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 nervous centre from which the sensitive 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 nerve be 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. When the cause of the neuralgia 

 is seated in the trunk of the nerve for example, of the facial 

 or infraorbital nerve division of the branches can be of no 

 service; for the stump remaining in connection with the brain, 

 and containing all the fibres distributed in the branches of the 

 nerve to the skin, continues to give rise, when irritated, to the 

 same sensations as are felt when the peripheral parts themselves 

 are affected. Division of a nerve prevents the possibility of 

 external impressions on the cutaneous extremities of its fibre 

 being felt; for these impressions can no longer be communi- 

 cated to the brain : but the same sensations which were before 

 produced by external impressions may arise from internal 

 causes. In the same way may be explained the fact, that when 



