STRUCTURE OF NERVE FIBRES. 159 



invertebrate animals, which all recent observers agree in describing as 

 consisting of fasciculi of fibrils with interfibrillar granular substance.* 

 Many Crustacea make exceptions to this, but only in so far that in 

 them a structure analogous to the medullary sheath is present, in the 

 interior of which fasciculi of fibrils lie enclosed, forming a kind of axis 

 cylinder.! 



Since their first discovery by Remak, the axis cylinders of the medul- 

 lated nerve fibres of man and other vertebrates have repeatedly been 

 held to exhibit a fibrillar structure. Remak himself described the 

 axis cylinder, or as he termed it, believing it to be hollow, the axis 

 tube, as marked by parallel lines, and regarded this as an indication 

 of its fibrous nature. J His followers, however, became more and 

 more convinced that the axis cylinder was a homogeneous structure, 

 and this has recently been maintained by Waldeyer, to whom we are 

 indebted for a laborious work on the subject. Waldeyer admits the 

 probability of the origin of the axis cylinder from isolated fibrils in the 

 nerve centres, just as he acknowledges that it splits peripherically into 

 fibrils, but he holds that in its course it is a homogeneous structure. 



Kolliker has arrived at the same result, since after adducing nu- 

 merous arguments in favour of the fibrillar nature of the axis 

 cylinder, he concludes with these words :. " There is no absolute and 

 decisive proof of fibrillation in the axis cylinder. "H 



I am very far from denying that the axis cylinder, as it is ordinarily 

 brought into view, gives the impression rather of a homogeneous than 

 of a fibrillated cord. There is no doubt that when examined with 

 moderate powers, and when hardened by the ordinary methods, its 

 substance does appear homogeneous, or presents only a linear arrange- 

 ment of fine molecules. But in proportion as the process of har- 

 dening is avoided in the prosecution of the investigation, and the 

 structures are maintained, both as regards their consistence and 

 refractive powers, in a state analogous to the fresh condition, es- 

 pecially when high magnifying powers are employed, so much the more 

 clearly am I able to recognise a parallel striation and a, substance of a 

 finely granular nature between the striae, which are appearances that 

 I can only refer to the axis cylinder being constructed of fibrils, and 



* Cf. especially Leydig, Lehrbuch der Hislologie des Menschen und der 

 Thiere, 1857. 



t Remak and Hackel, the last in Miiller's Archiv, 1857, p. 469. 

 \ Observations Anatomicce, etc., 1838, p. 2, note 2. 

 Zeitschrift fur rationelle Medicin, Band xx., 1863, p. 193. 

 \\ Gewebelehre, fifth Aufl., 1867, p. 244. 



