70 CLARKE, ON NERVE-FIBRE. 



The medullary sheath, or white semi-fluid, is semi-fluid 

 and exceedingly pellucid, of great refrangibility, extremely 

 extensible, but inelastic, and of a peculiarly viscid nature, so 

 that when its continuity is interrupted, or whenever it is in 

 any way disturbed, it has little or no tendency to return to 

 its original position ; and, like other semi-fluid and viscid 

 substances, may be drawn into fibres or into delicate expan- 

 sions of extreme tenuity. In its natural position around the 

 axis-cylinder (PI. IV, fig. a), its outer and inner surfaces (b, c), 

 where they are seen side by side at the lateral parts of the 

 fibre, give rise respectively to the outer and inner contour or 

 line ; but if it constituted the entire fibre, instead of only a 

 layer around the axis-cylinder (a), it is evident that it would 

 be bounded only by a single, outer contour, or dark line, on 

 each side. Now a fold or ridge (fig. 1) raised up from the 

 white substance presents to an eye looking down on its 

 convex surface the same appearance as such a fibre would 

 present; that is, it appears as a fine tubule or fibre bounded 

 on each side by a single dark outline. "When I directed my 

 attention to nerves hardened in chromic acid, with the 

 special view of ascertaining the cause of the fibroid appear- 

 ance, I soon became convinced, from many reasons, that 

 these so-called fibres are produced in the way I have just 

 stated. The first peculiarity likely to strike an observer is 

 the unnatural thickness of the layer of white substance, and 

 consequently of the entire fibre (fig. 2). This increase in 

 diameter would of course be attributed by Stilling to the 

 separation from each other of the so-called " elementary 

 tubules," which on that account have a more striking 

 resemblance, at first sight, to actual fibres. But if these 

 were really fibres, there would be vacant spaces between 

 them, whereas they are all connected together by inter- 

 vening portions of the hardened and brittle, but extremely 

 transparent white substance. Nothing at first sight can 

 look more like fibres or tubules than the loops at a a, fig. 2 ; 

 and yet, on closer examination, they were found to be only 

 the rounded borders of portions of the white substance pro- 

 jecting from the surface; for they inclose a transparent 

 layer which connects their edges with the rest of the primi- 

 tive-fibre. On the opposite side, at b, is seen what might 

 easily be mistaken for a broken fibre; but it is nothing more 

 than the broken edge of a projection similar to those at a a, 

 with a transparent and almost imperceptible layer of white 

 substance connecting it with the general surface. In fig. 3, 

 at a, the same appearance is still more satisfactorily ex- 

 plained ; for here a piece of the rounded border of the pro- 



