128 NERVOUS TISSUE. 



the brain and nerves, and it was accordingly named the wliite svlstance 

 by Schwann, and by others, though less a])propriately, the mediiUarij 

 shrafh. 



Wliite substance of Schwann or Medullary Sheath. — It is 



this substance, which appears to be of a fatty nature, that undergoes the 

 most marked change on exposure ; it then seems to suffer a sort of coagu- 

 lation or congelation, and when this has taken place, it very strongly 

 refracts the light, and gives rise to the appearance of a dark border on 

 each side of "the nerve-tube (fig. 81, a and c). This border, though 

 darker than the rest of the tube, is nevertheless translucent ; it is bounded 

 by two nearly parallel lines, so that the nerve-fibre has a double con- 

 tour. The dark contours pursue a sinuous course, often with deep and 

 irreo'ular indentations ; while straight or curved linos of the same cha- 

 racter, occasioned no doubt by wrinkles or creases occurring in the layer 

 of white substance, are frequently seen crossing the tube. By continued 

 exposure, round and irregular spots appear at various points, and at 

 length the contents of the nerve-tube acquire a confusedly curdled or 

 granulated aspect. 



The double contour appears only in fibres of a certain size ; in fine fibres, 

 which become varicose or dilated at intervals, the double line is seen only in the 

 enlargements, and not in the narrow parts between. It often happens that the 

 soft contents of the tube are pressed out at the ruptured extremities, as in 

 fio-. 81. c, 4, and then the roimd or irregular masses of the effused matter are 

 stUl sun-ounded by a double contour, which proves that this appearance is pro- 

 duced independently of the membranous tube. 



The fine transparent membranous tube, named the primitive 

 sheath, or sheath of Schwann, presents an apparently homogeneous 

 appearance, with nuclei disposed at intervals along its inner surface. 



So long as tliis tube is accurately filled by the contained matter, its 

 outline can seldom l)e distinguished ; but sometimes, when the white 

 substance separates at various points from the inside of the tube, the 

 contour of the fibre becomes indented and irregular, and then the mem- 

 brane of the tul3e may, in favourable circumstances, be discerned as an 

 extremely faint line, running outside the deeply shaded border formed 

 by the white substance, and taking no part in its irregular sinuosi- 

 ties (fig. 81, c. 1, 1). The membranous tube may also be distin- 

 guished at parts where the continuity of the contained matter is broken 

 in consequence of traction, squeezing, or like injury of the fibre ; in 

 such parts the double line produced by the white substance is wanting, 

 and the faint outline of the membranous tube may be perceived passing- 

 over the interruption (2). The primitive sheath is not found on 

 fibres within the substance of the brain and spinal cord. 



The axis-cylinder, axis-band, or axial-fihre is situated in, or near, 

 the middle of the nerve-tube, where it may occasionally be seen, on a 

 careful inspection, as a greyish stripe or band, bounded on each side by 

 a very faint even outline, having no share in the sinuosities of the white 

 substance (fig. 81, c). 



The axis is of a more tenacious consistence than the white substance, 

 and may accordingly be sometimes seen projecting beyond it at the end 

 of a broken nerve-tube, either quite denuded, or covered only by the 

 tubular membrane, the intervening white substance having esca])ed. 

 In the brain and spinal cord it is especially easy to obtain this appear- 



