360 DISEASES OF THE PERIPHERAL NERVES. 



througn the canal of Fallopius in caries of the petrous bone, may be 

 regarded as a prototype of this variety. A peripheral end of a nerve 

 may also be separated from its central organ by the continued pressure 

 of a tumor, an aneurism, or an exostosis which gradually consumes it. 

 Sometimes even a temporary pressure upon a nerve has the same ef- 

 fect, which is probably owing to the continuity of the nerve's having 

 become broken at the compressed point. Thus JSasse has seen a mo- 

 tor palsy of the arm which proved refractory to all treatment, in a per- 

 son who had slept some time with his arm resting over the back of a 

 chair. In very rare cases the disturbance of continuity of the nerve is 

 due to a primary partial neuritis. 



The structural changes whereby the motor nerves lose their excita- 

 bility, without undergoing solution of continuity, are as unknown as are 

 those which destroy the function of the sensory nerves. Thus cutting 

 off the supply of arterial blood will arrest the irritability of the motor 

 nerves without producing in them any structural alteration. It is 

 probable, although not proved, that rheumatic palsy is due to hyper- 

 aemia and oedema of the neurilemma, causing compression of the nerve- 

 fibres. Physiological experiment warrants the supposition that the 

 paralysis, which sometimes arises after violent attacks of cramp, is 

 the result of over-exertion ; but a nerve which has lost its excitability 

 through undue exertion does not differ appreciably from an excitable 

 nerve. It is the same in case of a paralysis arising in consequence of 

 a moderate pressure or strain. Here, from its favorable course, we 

 must attribute the palsy to a diminution of the irritability of the nerve, 

 and not to its destruction. Finally, the structural changes of the 

 nerves are unknown, 'which give rise to the paralysis of lead-poison- 

 .ing and of miasmatic poisoning, as well as in the so-called essentia 

 palsies. (See Chapter XXI.) 



Although hysterical palsy has not been counted either among the 

 cerebral diseases or among those of the spinal marrow, it probably 

 does not belong to the class of peripheral paralyses, but rather de- 

 pends upon some impalpable anatomical abnormity of the centre of 

 volition, as we shall explain more in detail when treating of hysteria. 



SYMPTOMS AND COURSE. Complete separation of a nerve from 

 its central organ renders the muscles which it supplies incapable of 

 motion, producing complete palsy or paralysis. Textural changes, 

 which absolutely destroy the irritability of the nerve, have the same 

 effect. When the nervous irritability is merely diminished, but not 

 annihilated, feeble contractions still remain possible. This condition 

 is called incomplete palsy or paresis. 



A paralysis or paresis of peripheral origin is usually distinguishable 

 from a central palsy : 1. By its extent. As we have seen, the charac- 



