344 DISEASES OF THE PERIPHERAL NERVES. 



its irritability, is often so subtle in character as to escape our direct 

 observation. The moment a nerve loses its supply of arterial oxy- 

 genated blood, it loses its functional power. There is no doubt but 

 that this loss of irritability is due to physical or to chemical changes 

 within the nerve. Not unfrequently, in the region of an artery oc- 

 cluded by an embolus or thrombosis, we find anaesthesia to exist until 

 the collateral circulation has been established. The action of continued 

 cold induces anaesthesia of the skin. Part of this is undoubtedly due 

 to the contractile power of cold upon the skin, and to the anaemia of 

 the cutaneous nerves thus produced. Of the anaesthesiae of peripheral 

 origin that is, depending upon extinguished irritability of periph* 

 eral nerves, without appreciable alteration of their structure there is 

 the so-called rheumatic anaesthesia, which not unfrequently takes place 

 after exposure to cold, as well as a form which Homberg has noticed 

 in the hands and forearms of washerwomen. It is doubtful whether 

 the anaesthesia arising from lead-poisoning be due to impalpable nutri- 

 tive lesion of the peripheral nerves, or to a morbid state of the central 

 organs. This is also true of the locally anaesthetic action imputed to 

 ether and chloroform. It does not seem to me to be proved that the 

 action of these substances is purely local, when locally applied ; and I 

 think it more likely that their influence is more or less centric at such 

 times, if not entirely so. Can we expect to find a perfect anaesthetic 

 action from such agents, the cerebral function remaining intact mean- 

 time ? It bears a peculiar relation to hysterical anaesthesia, of which 

 we shall treat in our chapter upon hysteria. 



Anaesthesia due to permanent and uniform pressure of an over- 

 tight bandage or piece of clothing, constitutes a connecting link with 

 those forms of anaesthesia where structural lesion of the peripheral 

 nerves is not merely a matter of supposition, but can be absolutely 

 demonstrated. In such cases we find all the tissues, subjected to pres- 

 sure, to be in a state of atrophy and fatty degeneration, and, of course, 

 the peripheral nerves are by no means exempt. The irritability of a 

 nerve is not unfrequently suspended, and its continuity is finally inter- 

 rupted by the pressure of a tumor, an exudation, or extravasation. In 

 other cases of anaesthesia, portions of the nerve are destroyed by in- 

 flammation. It would be superfluous further to continue the enumera- 

 tion of the structural alterations capable of producing anaesthesia, as 

 it would be but a repetition of what we have already stated in treat- 

 ing of the etiology of neuralgia. The very causes which, excited mildly, 

 or for a short time, serve to exalt the excitability of a nerve, have a 

 more pernicious effect should their action be prolonged or intensified, 

 as the nervous excitability is then arrested, or even the nerve itself 

 may be destroyed. 



