508 



NERVOUS SYSTEM. 



strictions at various points in their length, called the constrictions or nodes 

 of Eanvier. At these nodes the medullary substance is wanting and the 



neurilemma is in contact with the axis- 

 cylinder. It is at these points that the 

 transverse lines of Fromann, produced by 

 the action of silver nitrate upon the axis- 

 cylinder, are particularly prominent. 



When a medullated nerve - fibre is 

 slightly stretched, a number of oblique 

 cuts are observed running across the fibre 

 and extending to the axis-cylinder, called 

 incisures. These involve the medullary 

 substance only, and are best observed when 

 this substance has been stained with os- 

 mic acid. It is not knoAvn that they pos- 

 sess any physiological importance. 



The axis-cylinder, which occupies one- 



FIG. ^.-Nerve-fibres from the human sub- fifth to one-f ourth of the diameter of the 

 ject; magnified wdiater*(Kviuker). nerv e-tube, is probably the conducting 



In the ordinary 



Four small fibres of which two are varicose. 



one medium-sized fibre with borders of portion of the nerve, 

 single contour, and four large fibres. Of * 



the latter, two have a double contour, and medullated fibres, the axis-cylinder can 



two contain granular matter. i . . 



not be seen in the natural condition, be- 

 cause it refracts in the same manner as the medullary substance; and it 

 can not easily be demonstrated afterward, on account of the opacity of the 

 coagulated matter. If a fresh nerve, however, be treated with strong acetic 

 acid, the divided ends of the fibres retract, leaving the axis-cylinder, which 

 latter is but slightly affected by reagents. It then presents itself in the 

 form of a pale, slightly flattened band, with outlines tolerably regular, 

 though slightly varicose at intervals. It is somewhat granular and very finely 

 striated in a longitudinal direction. This band is elastic but not very resist- 

 ing. What serves to distinguish it from all other portions of the nerve-fibre 

 is its insolubility in most of the reagents employed in anatomical investiga- 

 tions. It is slightly swollen by acetic acid but is dissolved after prolonged 

 boiling. If nerve-tissue be treated with a solution of carmine, the axis-cyl- 

 inder only is colored. It has been observed that the nerve-fibres treated with 

 silver nitrate present in the axis-cylinder well marked, transverse striations 

 (Fromann) ; and some anatomists regard both the nerve-cells and the axes 

 of the fibres as composed of two substances, the limits of which are marked 

 by the regular striae thus developed. This, however, is a point of purely 

 anatomical interest. The presence of regular and well marked striae in the 

 axis-cylinder after the addition of a solution of silver nitrate and the action 

 of light can not be doubted ; but it has not yet been determined whether 

 these markings be entirely artificial or whether the axis-cylinder be really 

 composed of two kinds of substance. 



For some time it has been known that the axis-cylinders in the organs of 

 special sense, in the final distribution of sensory nerves and in some other 



