396 SPECIAL HISTOLOGY. 



geneous. I have endeavoured to the utmost of my power to 

 investigate the relations of this structure, and have arrived at 

 the following results : 



1. The axis -cylinder, is constantly present in every nerve- 

 fibre, both central and peripheral, in fine and in coarse fibres, 

 and after death is apparent before the nerves are treated with 

 any reagent whatsoever. In the human nerves, in the brain and 

 spinal cord, as they are commonly obtained for examination, 

 the axis-cylinder, when duly sought for, is everywhere and with 

 certainty to be recognised ; and in fact by far the most easily 

 in the central organs, where the absence of neurilemma and 

 the delicacy of the nerve-sheaths oppose but little hindrance to 

 the tearing asunder of the fibres. In these situations it may he 

 seen in nearly the finest fibres. It always presents the aspect of 

 a pale filament, which, together with a tolerable degree of con- 

 sistence, is still very flexible and at the same time highly elastic, 

 as may be readily observed on compression of small portions of 

 the spinal cord (in which case very many axis-cylinders are 

 stretched and torn, retracting considerably and forming undu- 

 lating curves). On the average it is about one third as wide as 

 its nerve-fibre, and consequently varies a good deal in diameter, 

 is obviously quite solid, most generally homogeneous, but not 

 unfrequently also, faintly striated or very finely granular. It 

 most usually follows a straight course, bordered by two parallel, 

 pale contour lines, occasionally, however, it is, in parts, thicker 

 or more slender, though it never presents varicosities like the 

 nerve-fibres; and it may, moreover, be curved or even slightly 

 undulating, and also perhaps with an irregular, even jagged 

 border. 



2. When recent nerve-fibres of an animal just killed are treated 

 with proper reagents, the axis-fibre instantaneously appears. If a 

 thin cutaneous nerve of the Frog, whilst under examination 

 with a power magnifying 100 times — be touched with a drop 

 of glacial or concentrated acetic acid, the nerve retracts and 

 there appear instantaneously, at each of the cut ends, large 

 particles of the grumous nerve-pulp, and pale, clear fibres ; and 

 the same thing happens if the nerve have been previously teased 

 out, and the fibres brought separately into view. The clear 

 fibres are evidently the axis-fibres, as they may readily be traced 

 into the projecting medullary sheaths and entire nerve-tubes, 



