PROGRESS OF MICROSCOPICAL SCIENCE. 229 



appearance of clear, higlily-refracting biconcave disks, seen in profile 

 and placed across the long axis of tlie nerve. On carefnl examination 

 each disk is found to be divided into two symmetrical halves by a 

 transverse line of extreme fineness ; either half of the disk belongs to 

 the corresponding nerve-segment, and may be traced uninterruptedly 

 into its Schwannian sheath and the protoplasm by which the same is 

 lined. The septa thus formed between the individual segments are so 

 far complete that, as has been already mentioned, they entirely 

 separate the medullary sheath of neighbouring segments from each 

 other, and make the medullary sheath of a nerve-tube not a continuous 

 but a regularly interrupted covering. The axis cylinders of the seg- 

 ment, on the other hand, are all perfectly continuous ; they pass 

 iiuinterruptedly through a nearly central opening in the inter- 

 segmental disk, and thus there is a single unbroken conducting axis 

 of nervous matter in each tube. The length of each segment while 

 constant in a given nerve is decidedly less in a young than in an adult 

 animal — that is, in a growing nerve than in a fully formed one ; and 

 Eanvier makes the important observation that newly-developed portions 

 of nerves might thus be recognized in a healing wound. 



The function of the annular constrictions in nerves is very evident. 

 The fatty material of which the medullary sheath is composed is not 

 permeable by the nutritive fluids ; and it is only through these inter- 

 ruptions in the medullary sheath that the axis cylinder can possibly 

 be nourished. 



Eanvier next investigated the histology of the connective tissue 

 aroimd the nerves.* The most interesting points which he made out 

 related to the structure of the sheaths immediately surrounding the pri- 

 mary bundles of nerve-fibres. These primary sheaths are composed of 

 concentric lamellfe of a homogeneous elastic substance, in which bundles 

 of connective tissues are disposed, the whole forming a covering of 

 remarkable strength for the bundle of nerves which is enclosed. This 

 explains the great resistance to suppui'ation and ulceration which nerves 

 have always been known to possess. However, there is a limit even to 

 this resistance. If the sciatic nerve of a living rabbit is laid bare', and 

 water allowed to fall upon it drop by di'op, paralysis of the corre- 

 sponding muscles will follow in fifteen to eighteen minutes ; and if an 

 examination of the nerve be made at once, a remarkable alteration will 

 be found to have taken place upon the fibres within the sheath, for the 

 annular constrictions have disaj)i)eared and the whole nerve is swollen, 

 especially the axis cylinder. In forty-eight hours the fibres have 

 completely degenerated. From this observation Eanvier di-aws the 

 practical conclusion that irrigation of a wound in which nerves are 

 exposed may not be so harmless as is generally supposed. 



In his third and last research, Eanvier made a practical applica- 

 tion of the knowledge which he had acquired to the investigation of 

 the changes undergone by a nerve after section. f The changes upon 

 the central and peripheral ends of the cut nerve are remarkably dif- 

 ferent. While the central extremity presents merely a granular 

 degeneration, and its axis cylinder remains uninterrupted, the peri- 



* Ibid., July, 1872. 



t ' Comptes Rendus,' December 30, 1872, No. 72. 



