

ON THE NERVES IN GENERAL. 495 



large scale, is seen on a small in each nerve; and the cords 

 themselves are merely plexuses of nervous filaments. Towards 

 the origin or the central extremity of the nerves, the neurile- 

 ma is continuous with the pia-mater, but only in that portion 

 of it which constitutes the general envelope of the nerve: the 

 interior sheaths of the nervous filaments become softened and 

 lost insensibly, so that these are naked in the centre of the 

 nerve. The nerves" equall} 7 are seen to become deprived of 

 their neurilema at their termination, wherever they can be 

 traced far enough. The neurilematic canals do not present in 

 the interior a smooth and polished surface as is the internal 

 surface of vessels; they give out a multitude of prolongations 

 which traverse the medulla of the nerve and sustain it: this 

 last is not free and moveable in the nerve which it owes part- 

 ly to its consistence, but which is also owing in part to this 

 disposition. There exists cellular tissue about the general 

 sheath, aad between the partial sheaths of the nerve, as has 

 been observed with respect to the muscular fascicles and the 

 fibres of which they are composed. In neuralgia, this tissue 

 Is sometimes the seat of an oedema and of an infiltration which 

 renders it, in certain cases, compact and close; at other times 

 of a sanguineous congestion or of a very great redness, as 

 Cotugno and others have observed, which leads to the opinion 

 that these painful affections depend on its inflammation. Fat 

 may also accumulate in this tissue. The medullary fibres, 

 contained in the canals of the neurilema, are of the same na- 

 ture with those of the brain and spinal marrow. 



780. The blood" vessels of the nerves penetrate between 

 the cords of which they are composed, and divide, for the 

 most part, into two branches, one direct, the other retrograde. 

 Their number is considerable: all the neurilema is perceived to 

 be covered with them in .. successful injections; they are seen 

 with a lens to spread over even the neurilema of the nervous 

 filaments. This last is formed of fibrous cellular tissue and of 

 blood vessels. No lymphatic vessels of the nerves are known. 



781. The structure of the nerves is not exactly the same 

 in all. In the greater part of the researches which have been 



made on this subject, the optic nerve has been chosen, because 

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