MOTOR AND SENSORY NERVES. 525 



cles of the eyes, is due mainly to habit and may be greatly modified by edu- 

 cation. 



Mode of Action of the Sensory Nerves. The sensory nerve-fibres, like 

 the fibres of the motor nerves, are entirely independent of each other in 

 their action ; and in the so-called anastomoses that take place between 

 sensory nerves, the fibres assume no new relations, except as regards con- 

 tiguity. 



As motor fibres convey to their peripheral distribution the impulse pro- 

 duced by a stimulus applied in any portion of their course, so an impression 

 made upon a sensory nerve is always referred to the periphery. A familiar 

 example of this is afforded by the very common accident of contusion of the 

 ulnar nerve as it passes between the olecranon and the condyle of the hu- 

 merus. This is attended with painful tingling of the ring and little finger and 

 other parts to which the filaments of this nerve are distributed, without, 

 necessarily, any pain at the point of injury. More striking examples are 

 afforded in neuralgic affections dependent upon disease of or pressure upon 

 the, trunk of a sensory nerve. In such cases, excision of the nerve is often 

 practised, but no permanent relief follows unless the section be made be- 

 tween the affected portion of the nerve and the nerve-centres ; and the pain 

 is always referred to the termination of the nerve, even after it has been 

 divided between the seat of the disease and the periphery, leaving the parts 

 supplied by the nerve insensible to direct irritation. In cases of disease it 

 is not unusual to note great pain in parts of the skin that are insensible to 

 direct impressions. The explanation of this is that the nerves are paralyzed 

 near their terminal distribution, so that an impression made upon the skin 

 can not be conveyed to the sensorium ; but the trunks of the nerves still 

 retain their conducting power and are the seat of diseased action, producing 

 pain which is referred by the patient to the periphery. In the very common 

 operation of restoring the nose by transplanting skin from the forehead, 

 after the operation has been completed, the skin having been entirely sepa- 

 rated, and united in its new relations, the patient feels that the forehead is 

 touched when the finger is applied to the artificial nose. After a time, how- 

 ever, the sensorium becomes accustomed to the new arrangement of the 

 parts, and this deceptive feeling disappears. 



There are certain curious nervous phenomena, that are not without physi- 

 ological interest, presented in persons who have suffered amputations. It 

 has long been observed that after loss of a limb, the sensation of the part re-- 

 mains ; and pain is frequently experienced, which is referred to the ampu- 

 tated member. Thus a patient will feel distinctly the fingers or toes after 

 an arm or a leg has been removed, and irritation of the ends of the nerves at 

 the stump produces sensations referred to the missing member. After a 

 time the sense of presence of the lost limb becomes blunted, and it may in 

 some cases entirely disappear. This may take place a few months after the 

 amputation or the sensations may remain for years. Examples have been 

 reported by Muller, in which the sense was undiminished thirteen, and in 

 one case, twenty years after amputation. In a certain number of cases, 



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