ANESTHESIA OF THE CUTANEOUS NERVES. 343 



form of anaesthesia, which we have mentioned as one of the symptoms 

 of many of the diseases of the brain, as this section of the work is to 

 be devoted to a study of diseases of the peripheral nerves. 



2. Anaesthesia may occur when, owing to destruction of the con- 

 ducting fibres of the spinal cord, transmission of the impressions from 

 the peripheral nerves to the brain is interrupted. In this form, like- 

 wise, the excitability of the peripheral nerves may be normal. In 

 treating of myelitis, we have already mentioned that, while the stronger 

 stimulants, acting upon the periphery, fail to call forth any excitement 

 at the centre, even a very feeble stimulus is frequently transmitted 

 from the sensory to the motor fibres, and gives rise to reflex symptoms. 

 Such phenomena as these prove that the excitability, both of the motor 

 and sensory nerves, still remains unimpaired below the point at which 

 the conducting power is interrupted. This form of anaesthesia, also, 

 is not the subject of the present section. 



3. Anaesthesia occurs when there is nutritive disorder capable of 

 destroying the irritability of the peripheral nerves, as well as when 

 their connection with the brain and spinal cord has been interrupted 

 mechanically. It is with this form alone that we are at present con- 

 cerned. We shall here observe that we consider all sensory and motor 

 nerves as peripheral as soon as they leave the brain, no matter whether 

 they continue to lie within the skull or spinal canal, or whether their 

 track be outside of the same. This is by no means an arbitrary dis- 

 tinction. It depends chiefly upon the fact that the portion of nerve 

 lying within the skull or spinal canal is in just the same state as 

 the peripheral portions, as regards extinction of its irritability upon 

 interruption of its connection with the central organs. In the 

 motor nerves this can be proved with ease. In a patient who is 

 unable to move the right side of his face at will, owing to an apo- 

 plexy of the left corpus striatum, the nerves of the palsied side retain 

 their irritability for weeks, and all the muscles of that side may be 

 made to contract under the influence of the induced electric current. 

 On the other hand, if the facial be destroyed within the skull after its 

 departure from the brain, its irritability soon becomes extinct, just as 

 though its peripheral branches had been divided, and it is impossible 

 to make the muscles of the palsied side contract by the influence of 

 the induced current. With regard to the sensitive nerves, the proof 

 cannot be made so plain, yet we are warranted in inferring that the 

 conditions are very similar. At all events, in central anaesthesia, the 

 trigeminus long remains sensitive, as is proved by the continuance of 

 the power of reflex action, for instance, as is shown in winking when 

 the conjunctiva is touched. 



ETIOLOGY. The nutritive disorder, whereby a nerve is deprived of 

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