THE NERVOUS SYSTEM. 347 



viewed, transparent and clear, or whitish and pearly, and it is obviously 

 to it that the peculiar glistening appearance of the nerves is due. The 

 nerve pulp is rapidly and invariably altered by the application of cold 

 water, of most acids, and of many other reagents, the change consist- 

 ing principally in a coagulation of it, which takes place gradually from 

 without to within, sometimes involving the entire thickness, sometimes 

 only its outermost layer. In the latter case, are produced the well- 

 known nerve-fibres with double contour lines (Fig. 137, 2, 3, 4), or in 

 which the medullary sheath is, externally, coagulated to a greater or less 

 extent, remaining fluid internally ; in the former case, with the contents 

 apparently wholly grumous and opaque (Fig. 138). The coagulated nerve- 

 medulla, in fact, seldom appears homogeneous, but most generally gru- 

 mous, granular, and as if composed of separate, irregular, larger and 

 smaller masses, and, upon the application of acetic acid, as if formed of 

 minute, separate, or reticularly united rods. The nerve-pulp is also 

 altered very readily by pressure. It sometimes escapes from the ends 

 of the tubes, or from hernial protrusions or ruptures of the sheath, form- 

 ing larger or smaller drops of every imaginable shape, regularly spherical, 

 clavate, fusiform, cylindrical, filamentary, or of the most bizarre figures, 

 which likewise coagulate either on the surface merely, or throughout, 

 and thence, like the nerves, appear with a double contour, or half or 

 wholly grumous. But, within the fibres also, its structural conditions 

 alter, for, instead of being continued through them as before, as a 

 cylinder of uniform size, it accumulates in places into larger masses. 

 In this way are produced the frequently described, varicose nerve-fibres 

 (Fig. 138), in which the medullary sheath presents sometimes, minute 

 moniliform enlargements, sometimes, various sized, irregularly distri- 

 buted nodosities, or even, in places, complete interruptions. All these 

 forms, in which the sheath frequently participates, but in which the cen- 

 tral fibre takes no part, arise artificially, and are developed most readily 

 in the finer fibres with more delicate sheaths, such as are found in the 

 central organs. 



The central or axis-fibre of the nerve-tubes ("primitive band," Re- 

 mak, cylinder axis, Purkinje, Fig. 137, 2, 3, 4, 5 ; 139, 1), is a cylin- 

 drical or slightly flattened filament, which, in entire and unaltered nerve- 

 fibres, is as little recognizable as the sheath, being surrounded by the pulp, 

 and possessing the same refractive power, whilst it comes readily into 

 view when the fibre is torn or treated with various reagents ; and it may 

 thus be recognized as a constant structure, sometimes in the interior 

 of the fibre, arid sometimes isolated. Under natural circumstances it 

 is pale, most generally homogeneous, more rarely, finely granular or 

 striated, bordered by straight or occasionally by irregular, pale contour 

 lines, and it is, generally, everywhere of uniform thickness : it is distin- 

 guished from the medullary sheath, especially by the circumstance, that 



