82 HAND-BOOK OF PHYSIOLOGY. 



Illustrations. -t-li is in accordance with this law, that when parts are 

 deprived of sensibility by compression or division of the nerves supplying 

 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 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 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 

 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. In the same way may be ex- 

 plained the fact, that when part of a limb has been removed 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 re- 

 moved. It is not quite certain whether those sensations are due to con- 

 duction through the nerve fibres which are on their way to be distributed 

 elsewhere, or through the sentient extremities of nerves which are them- 

 selves distributed to the many trunks of the nerves, the nervi nervorum. 

 The latter is the more probable supposition. 



When, in a part of the body which receives two sensory nerves, one is 

 paralyzed, the other may or may not be inadequate to maintain the sensi- 

 bility of the entire part; the extent to which the sensibility is preserved 

 corresponding probably with the number of the fibres unaffected by the 

 paralysis. There are instances in which the trunk of the chief sensory 

 nerve supplied to a part having been divided, the sensibility of the part 

 is still preserved by intercommunicating fibres from a neighboring nerve- 

 trunk. 



Conduction in the Nerves of Special Sense. The laws of con- 

 duction in the olfactory, optic, auditory, gustatory resemble in many 

 .aspects those of conduction in the nerves of common sensation, just de- 

 scribed. Thus the effect is always central; stimulation of the trunk of 

 the nerve produces the same effect as that of its extremities, and if the 



