THE NERVOUS SYSTEM. 351 



agreed with Remak and Purkinje in regarding the axis-cylinder as a 

 constant element in recent nerve, whilst most observers have adopted the 

 views of Valentin ("Report." 1838, p. 76, 1839, p. 79), and Henle 

 (" Allg. Anat."), who regard it as not always present, but rather as a 

 secondary formation, which does not exist during life, and as the un- 

 coagulated central portion of the contents of the nerve-fibre, which, 

 during life, are homogeneous. I have endeavored 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 

 everyivhere and with certainty to be recognized ; 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 be seen in 

 nearly the finest fibres. It always presents the aspect of a pale filament, 

 which, together with a tolerable degree of consistence, is still very flexible 

 and at the same time highly elastic, as may be readily observed on com- 

 pression of small portions of the spinal cord (in which case very many 

 axis -cylinders are stretched and torn, retracting considerably, and 

 forming undulating 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 unfre- 

 quently 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 irre- 

 gular, 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 cuta- 

 neous nerve of the Frog, whilst under examination with a power mag- 

 nifying 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 pre- 

 viously 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, and in 

 other respects, also, present all the characters of those fibres, only that 

 they are much paler and broader (as much as 0-004 of a line, in the 



