348 



SPECIAL HISTOLOGY. 



although soft and flexible, it is still, not fluid and viscous, but elastic 

 and solid, something like coagulated albumen, with which it also ap- 

 pears to agree in most of its chemical 

 characters. This axis-cylinder exists 

 in all nerve-fibres without exception, 

 even in the finest, and invariably pre- 

 sents the same characters, except 

 only, that it is sometimes thicker, 

 sometimes more slender, according 

 to the size of the fibre itself. 



The nerve-fibres, in Avhich the 

 three structures above described can 

 be distinguished, and which we would 

 designate as the medidlated 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 contents, sometimes identical with the axis-fibre of the other kind 

 of nerves, sometimes similar but more clear. These non-medullated 

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

 where the latter are in connection with nerve-cells ; and also as more 

 elongated, independent fibres, representing 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 subdivisions, 

 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 to the delicacy or firmness of their structure, and partly in their 

 diameter, which varies from 0*0005 to O'Ol of a line, 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, 



Fig. 139. — Nerve-fibres, magnified 350 diameters. 1, from the Frog, boiled with alco- 

 hol and acetic acid ; a, sheath ; b, axis-cylinder ; c, crystals (fat ?) : 2, isolated sheath of a 

 Frog's nerve, boiled with soda : 3, from the floor of the fourth ventricle in Man, after treat- 

 ment with soda ; a, sheath ; 6, medulla flowing out in drops, the axis-cylinder is wanting 

 (having been drawn out in the preparation), and the pale streak is medulla : 4, from the 

 root of the n. abducens of Man, treated with soda ; a, sheath ; b, medulla, axis-cylinder not 

 visible. 



