CAUSE OF STRIATION OF VOLUNTARY MUSCULAR TISSUE. 323 
this part of the work. Very fine emery powder should be 
sprinkled over the preparation before covering it; for then, on 
examination, numberless little black specks will be seen in the 
field. A cross band of a fibre is selected for examination which 
is exactly opposite one of these little specks, then when you 
rotate you can definitely affirm, having the little black speck for 
your guide, what change has occurred. 
Rabbits’ muscles are very satisfactory objects for examination, 
as they do not cleave across at all readily. The adductor muscles 
of the leg should be excised, slightly stretched on a piece of wood, 
and placed in 50 per cent. alcohol until they split readily into 
fibrils. They may then be mounted in any ordinary fluid, a pinch 
of emery powder having been sprinkled over the preparation 
before covering. 
It is necessary to use a power of 800 or 1000 diameters in the 
investigation of mammalian muscle, while in the case of the insect 
one of 800 diameters is quite enough. 
In the living and dead muscular fibre the whole of its substance is 
doubly refracting. The observations of some modern observers 
entirely agree with my own, in that with crossed Nicols the 
crests (dark bands) and the centres of the valleys (bright stripes) 
appear bright and therefore refract light doubly, and that there 
are two dark bands on the slopes between them (see diagram, 
p. 5, Fig. 2). It does not foilow, however, that these two dark 
bands represent tracts of isotropous substance. This is the point 
at issue. The dark lines between the valieys and ridges which 
appear when tie Nicols are crossed have been interpreted as 
marking the positions of cross bands of singly refracting sub- 
stance, but this is a fault of reasoning. Ifthe fibre were smooth 
and cylindrical it would then follow, but the fibre is not, as we 
have already insisted. These bands lie just on the sloped parts 
of the fibre; those sections, in fact, which are oblique to the 
passing rays; and the explanation is now quite easy, for the 
extraordinary ray passing through the fibre is naturally deflected 
at these parts, and does not reach the eye of the observer. 
Hence the body appears not to transmit them at all at these 
parts. 
It is not difficult to explain the discrepancies between Briicke’s 
description of the bright stripe and my own. 
It is essential to be very scrupulous in the selection of a fibre 
for examination. It must not be at all twisted, nor sheared in 
the slightest degree, for then the cross stripes are not at right 
angles to the long axis, and as their width 1s several times their 
thickness (in the length) overlapping will to some extent occur. 
This wilk certainly lead to very confusing results, and the bright 
centre of the bright stripe (valley) may well he overlooked, 
