SECT. 109.] NERVOUS SYSTEM. 21 5 



dition, a nerve of a newly-killed animal must be quickly brought under the 

 microscope without adding anything ; in which case single fibres are always 

 seen entirely unaltered, although they are very quickly altered by the drying 

 of the nerve. Besides, I would recommend the observation of the nerves in 

 the transparent parts of living or newly-killed animals (nictitating membrane, 

 mucous membrane of the frog, the tail of the tadpole, etc.), and their exami- 

 nation upon warmed plates of glass (Stark), and after treatment with chromic 

 acid, which maintains them, especially the cerebral nerves, often quite un- 

 injured. 



The central Jibre of the nerve-tubes is, in human nerves, in the brain and 

 spinal cord, as they are usually received for investigation, everywhere and 

 with certainty recognisable on close examination, and, indeed, most easily in 

 the central parts, where the absence of neurilemma and the delicateness of 

 the nerve-sheaths less oppose the tearing of the tubes. Moreover, the central 

 or axis-fibre may also be occasionally recognised in perfectly fresh nerves of 

 animals. If a thin cutaneous nerve of a newlydulled frog be touched with 

 a drop of concentrated or glacial acetic acid, while at the same time it is 

 observed with a power magnifying 100 times, it is instantaneously seen, that 

 whilst the nerve shortens, the pale, clear, axial fibre passes out at both the 

 cut ends, together with large portions of the medullary sheath, which has 

 become gruinous. Alcohol and ether, especially when hot, display the axis- 

 cylinder very beautifully. Besides the re-agents mentioned, chromic acid, 

 corrosive sublimate, iodine, or iodine with hydriodic acid, may be used with 

 advantage. Hydrochloric, sulphuric, and fuming nitric acid also bring the 

 axis-cylinder, in certain cases, to view. 



With reference to its chemical properties, the central fibre swells very 

 considerably in concentrated acetic acid, dissolves, however, with difficulty, 

 and, even after several minutes' boiling, although pale, is still unaltered. 

 When boiled longer with acetic acid, it dissolves just as coagulated albumen 

 does ; on the other hand, the sheaths and some of the contents remain 

 undissolved. Alkalies (potass, soda, ammonia), when cold, attack the axis- 

 cylinder but slowly, still it becomes pale instantaneously in caustic soda, and 

 swells up to o - oo4'", o - oo5'", or even o - oo6'" in diameter. Longer treatment in 

 caustic soda dissolves it ; and this takes place more rapidly with the aid of 

 heat ; as soon, in fact, as the fluid begins to boil. When treated with nitric 

 acid and caustic potass, the axis-cylinder becomes yellow (xanthoiiroteic acid), 

 and appears spirally contracted in the nerve-tube, which is also shortened, 

 although in a slighter degree. On the other hand, sugar and concentrated 

 sulphuric acid, which colour coagulated albumen red, do not colour the axis- 

 cylinder, or at most, give it a yellowish or a slightly reddish tint. In water it 

 is not altered, not even by boiling ; but by that means it is readily isolated, 

 and appears somewhat shrivelled ; ether and alcohol, even when boiling, do 

 not dissolve it, but cause it to shrink a little. The latter effect is produced 

 also by corrosive sublimate, chromic acid, iodine, and carbonate of potass. 

 Taking all these reactions together, it may, perhaps, be concluded with cer- 

 tainty, that the axis-cylinder is a coagulated protein compound, which, how- 

 ever, differs from fibrine, inasmuch as it does not dissolve in carbonate of 

 potass and nitric acid, and offers much more resistance to acetic acid and 

 caustic alkalies. It agrees with the substance wdiich constitutes the muscular 

 fibrils, in its elasticity and insolubility in carbonate of potass ; but is distin- 

 guished from it by its insolubility in diluted hydrochloric acid, and its difficult 



