NERVOUS EXCITABILITY AND CONDUCTIVITY 479 



common operation of restoring the nose by transplanting skin from the 

 forehead, after the operation has been completed, the skin having been 

 entirely separated, 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, however, 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 

 physiological interest, presented in persons who have suffered amputa- 

 tions. It has long been observed that after loss of a limb, the sensa- 

 tion of the part remains ; and pain is frequently experienced, which is 

 referred to the amputated member. Thus a patient will feel distinctly 

 the fingers or toes after an arm or a leg has been removed, and irrita- 

 tion 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 sensa- 

 tions may remain for years. Examples have been reported by Miiller, 

 in which the sense was undiminished thirteen, and in one case twenty, 

 years after amputation. In a certain number of cases, however, the sense 

 of the intermediate part is lost, the feeling in the hand or foot, as the 

 case may be, remaining as distinct as ever, the impression being that 

 the limb is gradually becoming shorter. It was noted by Gueniot, that 

 the sense of the limb becoming shorter exists in about half the cases of 

 amputation in which cicatrization goes on regularly ; and in these cases, 

 the patient finally experiences a feeling as though the hand or foot were 

 in direct contact with the stump. 



Physiological Differences between Motor and Sensory Nerve-fibres. 

 It has not been shown that there is any essential anatomical difference 

 between the conducting elements of motor and sensory nerve-fibres ; 

 but the physiological differences are sufficiently distinct, as has already 

 been seen. Under normal conditions, motor fibres conduct motor im- 

 pulses in but one direction, and these fibres are insensible. Sensory 

 fibres conduct impressions always in the opposite direction, and they do 

 not conduct motor impulses. 



Nervous Excitability and Conductivity. Immediately or soon after 

 death, when the excitability of the nerves is at its maximum, they may be 

 stimulated by mechanical, chemical or electric irritation, all these agents 

 producing contraction of the muscles to which the motor filaments are 

 distributed. Mechanical irritation simply pinching a portion of the 

 nerve, for example produces a single muscular contraction ; but if the 

 injury to the nerve is such as to disorganize its fibres, that portion of 

 the nerve will no longer conduct an impulse. Among the irritants of 



