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 PEOGEESS OF MICEOSCOPICAL SCIENCE. 



The Structure and Fierjeneration of Nerves. — We regret that the 

 article on this subject which appeared some time since in the ' Medical 

 Times and Gazette ' has not been earlier reproduced in our columns ; 

 but assuredly in the case of so important a communication " better 

 late than never " is perfectly excusable. Om* knowledge of the minute 

 structure of nerves has been considerably advanced by the recent 

 elaborate researches of Eanvier, who has sho^Ti that the description 

 of nerves hitherto given and accepted must now be modified in many 

 particulars. Eanvier undertook three series of investigations — the 

 first two upon the noi-mal histology of the nerve-tubes and their 

 sheaths ; and the third in application of the discoveries he had already 

 made, upon the changes which the nerves undergo after section. The 

 results obtained will be given in the same order. The subject of 

 Eanvier's first investigation * was the structure of the nerve-tubes, 

 nerve-fibres, .or primitive nerves, as they are variously named. An 

 ordinary medullated peripheral nerve-fibre is composed, as is well 

 known, of a protojilasmic axis cylinder, an insulating " white substance," 

 or medullary sheath, in which the former is imbedded, and a nucleated 

 membrane called the sheath of Schwann, which encloses the whole and 

 gives the nerve the strength and resistance for which it is remarkable. 

 We have hitherto believed that the nerve-tube is uniform in its entire 

 length — no transverse section of it being different from another. The 

 first important discovery made by Eanvier was that this description 

 must be considerably modified; that a medullated nerve is not a 

 uniform elongated structure, but that there occur upon it at regular 

 intervals peculiar annular constrictions, due in part to a complete 

 absence at these situations of the medullary sheath. This remarkable 

 condition Eanvier was first enabled to aiDpreciate by using some of the 

 rarer histological reagents in preparing the specimens, such as picro- 

 carminate of ammonia, perosmic acid, and nitrate of silver ; but once 

 the constrictions have been discovered and described, they may now 

 be recognized without difficulty, even in fresh nerves. A medullated 

 nerve-fibre must now be described as built up of segments exactly 

 similar in every respect, arranged end to end, and separated (or 

 united) by annular constrictions where their extremities come into 

 contact with each other. Each segment of the nerve is composed of 

 the three elements just enumerated — the axis cylinder, medullary 

 sheath, and sheath of Schwann, — but here also Eanvier's description 

 differs in some important respects fi'om what was previously given. 

 The Schwannian sheath of each segment is furnished with a single 

 nucleus only, and this nucleus lies exactly in the middle — i. e. at an 

 equal distance from the two ends — of the segment, and belongs rather 

 to a delicate layer of protoplasm lining the interior of the Schwannian 

 sheath than to the Schwannian sheath itself. The annular constrictions 

 which the nerve presents, or, as it may be otherwise expressed, the 

 planes by which the segments are united end to end, present the 



* ' Arcliiv. de Phys. Norm, et Path.,' March, 1872. 



