310 



MANUAL OF HISTOLOGY. 



system as well as in the olfactory nerves, as has just been mentioned, 

 pale, non-medullated, nucleated fibres are to be found 



But our position is far more difficult when we are 

 asked for an answer to the question, whether, in 

 what has been just described as the texture of the 

 nerve tubes, their whole structure has been given, or 

 whether they possess a further and more complex 

 constitution. 



For many years past there has been no lack of 

 efforts (and some very daring) to prove the latter to 

 be the case. Only one point, however, of any great 

 importance has been ascertained, through the im- 

 proved optical auxiliaries to our investigations, 

 namely, that the axis cylinder is made up of ex- 

 tremely delicate fibrillce, imbedded in a finely granular 

 substance. This fact was first recognised in the 

 pale nervous tubes of many invertebrate animals, and 

 in the olfactory nerve and fibres of Remak of the 

 vertebrates. It is also true for the axis cylinders of 

 the nervous centres (fig. 301), according to Schultze. 

 These extremely fine fibres, on which delicate vari- 

 cosities may be remarked after treatment with certain 

 reagents, have been named " axis fibrillas" by Wal- 

 det/er, and by Schultze " primitive fibrilla3." 



The axis cylinders of the stronger nerve tubes ap- 

 pearing thus as bundles of the most delicate fibres of 

 immeasurable fineness, those of less diameter must be 

 looked on as collections of smaller members of the 

 same, until eventually, in the most minute axis 

 cylinders, the number is reduced to one single pri- 

 mitive fibril (1). 



Later on we shall see that the primitive fibrilla (which call, however, 

 for closer observation as to their nature) make their appearance naked, 

 and separated one from the other in the termination of numerous fibres, 

 and also constitute important fibrous elements in the nervous centres. 



REMARKS. 1. The importance of the facts but briefly mentioned in the text entitles 

 them to more extended consideration. It was Itemak who first pointed out, years 

 ago, this remarkable complication in regard to the axis cylinder of the craw-fish. 

 In the garigliated cord of the latter are to be found, besides others, unusually thick 

 nerve fibres, whose axis cylinders consist of bundles of above a hundred of the finest 

 fibrillae, only 0'0004 mm. in diameter. This was subsequently corroborated \)y HaJcel, 

 Leydig, G. Walter, and Waldeyer, with discoveries of similar composition in the 

 nerves of other invertebrate animals. M. Schultze observed the same structure in the 

 axis cylinders of the olfactorius and nervous centres of vertebrates. As to the further 

 points of interest in regard to the structure of the nervous tubes, we have already 

 referred (p. 308) to the concentric markings to be seen in the medullary substance 

 on transverse section of the latter. It seems to depend on lamination, but this view 

 has been opposed by Frommann. According to JClebs, the axis cylinder is imme- 

 diately surrounded by a fluid substance, the "periaxial fluid." Years ago Stilling 

 also described as very complicated, the structure of the nerve fibre, working with very 

 high microscopic powers, and preparations made in chromic acid. Compare Lockhart 

 Clarke in the Quart. Journ. of M icrosc. Science, 1860, p. 165. More recently still 

 Frommann and Grandry have described a transverse striation on the axis cylinder 

 after treatment with nitrate of silver. RoudanowsTcy informs us further that the axis 

 cylinder is knotted, and gives off branches at right angles which anastomose with 

 those of neighbouring fibres. 



Fig. 301. Fibrillated 

 structure of the axis cy- 

 linder (after Schultze). 

 a, a strong axis cylinder 

 from the spinal cord of 

 the ox ; 6, a nerve fibre 

 from the brain of the 

 electric ray. 



