THE NERVOUS SYSTEM. 393 



is sometimes thicker, sometimes more slender, according to the 

 size of the fibre itself. 



The nerve-fibres in which the three structures above described 

 can be distinguished, and which we would designate as the 

 medullated or dark bordered, constitute, it is true, the greater 

 proportion of those existing in the body, but besides these there 

 are still some forms requiring more particular description. 

 These are the nerve-fibres in which there exists no trace of a 

 medullary sheath ; whilst they have an outer sheath and con- 

 tents, sometimes identical with the axis-fibre of the other kind of 

 nerves, sometimes similar but more clear. These non-mednl- 

 lated nerve-tubes occur, in the first place, as continuations to 

 the other sort, where the latter are in connexion with nerve- 

 cells ; and also as more elongated, independent fibres, represent- 

 ing the so-termed processes of the nerve-cells of authors; and 

 lastly, at the terminations of the dark bordered nerves. They 

 may again be arranged in several sub-divisions, distinguished 

 respectively by their having, or not having nuclei, and more or 

 less transparent, more or less consistent contents. It must 

 also be added, that the dark bordered fibres differ extremely — 

 partly in respect of the delicacy or firmness of their structure, 

 and partly in their diameter, which varies from 00005 — to 

 001'" or more, so that they may be distinguished into fine 

 and coarse, delicate or firm fibres ; from which it is evident 

 that the nerve-fibres, notwithstanding their general tubular 

 character, still differ pretty widely from each other in various 

 respects. 



[The tunic or sheath of the nerve-fibres, discovered by Schwann, 

 in most nerves is brought into view with some difficulty. It 

 is only rarely, as in the roots of certain cerebral nerves (those 

 of the muscles of the eye for instance), and of the spinal nerves 

 that it appears distinct from the contents; its presence how- 

 ever is, with certainty, and readily demonstrated by the aid of 

 chemical reagents. When the nerves are boiled in absolute 

 alcohol, soon after the removal of a considerable part of the fatty 

 matter of the pulp, the sheaths become tolerably distinct, as 

 dark boundary lines ; and they are rendered remarkably and 

 beautifully so by a short boiling in acetic acid, during which, 

 the remaining contents of the nerve-sheaths, with the exeep- 



