cxxxvi NERVOUS SYSTEM. 



and it is when so altered that they are commonly subjected to examination, 

 as represented in fig. LXXIV A. In particular instances, and in favourable cir- 

 cumstances, it may be discovered that the fibre is composed of a fine mem- 

 branous tube, inclosing a peculiar soft substance, and that this contained 

 substance itself is distinguishable into a central part placed like a sort of 

 axis in the middle of the tube, and a peripheral portion surrounding the 

 axis, and occupying the space between it and the tubular inclosing mem- 

 brane. In the annexed ideal plan (fig. LXXIV B), the membranous tube, or 

 primitive sheath, is marked 1, 1 : the central part, marked 3, was named 

 cylinder-axis by Purkinje, who considered it to be identical with the struc- 

 ture previously described by Remak under the name of the primitive band 

 (fibra primitiva) ; the matter surrounding it, marked 2, 2, is supposed to be 

 the chief cause of the whiteness of the brain and nerves, and it was accord- 

 ingly named the white substance by Schwann, and by others, though less 

 appropriately, the medullary sheath. It is this last-mentioned substance 

 which undergoes the most marked change on exposure ; it then seems to 

 suffer a sort of coagulation 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. LXXIV, A and c). This border, 

 though darker than the rest of the tube, is nevertheless translucent, and is 

 either colourless, or appears of a slightly yellowish or brownish tint ; it is 

 bounded by two nearly parallel lines, so that the nerve-fibre has then a 

 double contour, and the inner line, less regular than the outer, gradually 

 advances further inwards as the change in the white substance extends to a 

 greater depth. The dark contours pursue a sinuous course, often with deep 

 and irregular indentations ; while straight or curved lines 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 ex- 

 posure, 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 very fine tubes, 

 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 fig. LXXIV. 

 c, 4, and then the round or irregular masses of the effused matter p.re still surrounded 

 by the double line, which proves that this appearance is produced independently of 

 the membranous tube. So long as this tube is accurately filled by the contained 

 matter, its outline cannot be distinguished : but sometimes, when the white sub- 

 stance separates at various points from the inside of the tube, the contour of the fibre 

 becomes indented and irregular, and then the membrane of the tube may, in favour- 

 able 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 irre- 

 gular sinuosities (fig. LXXIV, c, 1. 1). The membranous tube may also be distinguished 

 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 fine transparent membrane 

 which forms this tube, named also the primitive sheath, appears to be quite simple 

 and homogeneous in structure ; so far as can be judged, it agrees in chemical nature 

 with elastic tissue. Treatment with weak chromic acid, or iodine, or, still better, 

 staining with carmine or aniline-red (magenta), brings into view nuclei attached 

 along the sheath. They are found on the fibres of the nerves generally, but not in 

 the optic and auditory nerves, nor in the brain and spinal cord ; and, indeed, it is 

 still a question whether the fibres of these last-named nerves and nervous centres are 

 provided with a membranous sheath. 



