THE MICROSCOPE AND ITS REVELATIONS. 



ultimate fibres of nerves, is a point not yet certainly ascertained. The 

 diameter of the fibres varies greatly in different kinds of Vertebrated 

 .animals. Its average is greater in Reptiles and Fishes than in Birds and 

 Mammals, and its extremes also are wider; thus its dimensions vary in 

 the Frog from l-100th to l-1000th of an inch, and in the Skate from 

 l-65th to l-300th; whilst in the Human subject the average is about 

 l-400th of an inch, and the extremes about l-200th and l-600th. 



678. The substance of the fibre, when broken up by 'teazing 3 with 

 needles, is found to consist of very minute fibrillae, which, when exam- 

 ined under a magnifying power of from 250 to 400 diameters, are seen to 

 present a slightly-beaded form, and to show the same alternation of light 

 .and dark spaces as when the fibrillse are united into fibres or into small 

 bundles (Fig. 473). The dark and light spaces are usually of nearly 

 equal length: each light space is divided by a transverse line, called 

 'Dobie's line;' while each dark space is crossed by a lighter band, known 

 as ' Hensen's stripe.' It has been generally supposed that these markings 

 indicate differences in the composition of the fibre; but Mr. J. B. Hay- 

 croft has recently revived an idea which originated with Mr. Bowman, 

 that they are the optical expressions of its sJiape. The borders of the 

 striated fibre (he truly states) present wavy margins, indicative of a trans- 

 verse ridging and furrowing; the whole fibre (or a single fibril) thus con- 

 sisting of a succession of convex bead-like projections with intermediate 

 concave depressions. When the axis of the fibre is in true focus, Dobie's 

 line, D, crosses the deepest part of the concavity, while Hensen's stripe, 

 H, crosses the most projecting part of the convexity; and it can be shown, 

 both theoretically and experimentally, that this alternation of lights and 

 shades will be produced by the passage of light through a similarly-shaped 

 homogenous rod of any transparent substance. If, on the other hand, 

 the surface of the fibre be brought into focus, the convex ribbings appear 

 light and the intervening depressions dark, which is the aspect origin- 

 allly represented by Bowman. The appearances are the same in the ex- 

 tended and contracted states of the fibre; with the exception that the 

 alternation of light and dark striae is closer in the contracted state, 

 while the breadth (representing the thickness) of the fibre is correspond- 

 ingly increased. 1 



679. In the examination of Muscular tissue, a small portion may be 

 cut-out with the curved scissors; this should be torn up into its compo- 

 nent fibres; and these, if possible, should be separated into their fibrillse, 

 by dissection with a pair of needles under the Simple Microscope. The 



feneral characters of the striated fibre are admirably shown in the large 

 bres of the Frog; and by selecting a portion in which these fibres spread 

 themselves out to unite with a broad tendinous expansion, they may often 

 be found so well. displayed in a single layer, as not only to exhibit all 

 their characters without any dissection, but also to show their mode of 

 connection with the ' simple fibrous ' tissue of which that expansion is 

 formed. As the ordinary characters of the fibre are but little altered by 

 boiling, recourse may be had to this process for their more ready separa'- 

 tion, especially in the case of the tongue. Dr. Beale recommends Gly- 

 cerine for the preparation, and Glycerine-media for the preservation, of 

 objects of this class; and states that the alternation of light and dark 

 spaces in the fibrillae is rendered more distinct by such treatment. The 

 fibnllae are often more readily separable when the muscle has been 



1 "Quart. Journ. Microsc. Science," N.S., Vol. xxi., p. 307. 



