436 
ME. E. A. SCHAFEE ON THE MINUTE STEUCTUEE OF 
line. It may be objected that if the rods were isotropous they would stand out as 
black markings on the bright ground : but this does not necessarily follow ; for since 
they are surrounded hy, and imbedded in, the anisotropous substance, any rays which 
reach them from the lower prism must of necessity pass through this substance, and 
thus be made available for illumination. 
In the contracted fibre the conditions are different ; for in this case the heads of 
the isotropous rods are enlarged and in close contiguity, so that there is no perceptible 
amount of anisotropous ground-substance between them ; and this being the case 
throughout the thickness of the fibre, we get a composite disk of isotropous substance 
produced, which remains dark between crossed Nicols. On the other hand, the anisotro- 
pous substance has now become collected between the shafts of the rods, disks of aniso- 
tropous substance being thus formed, which alternate and nearly correspond in size 
with those of isotropous. Consequently we have the effect produced of a series of 
illuminated stripes with dark intervals. The isotropous shafts of the muscle-rods, 
which are present in the former, do not stand out as dark streaks, for the same reason 
as that just stated in the consideration of the muscular fibre in the state of rest. 
Heppner* has given a somewhat similar account to that here given of the appearance 
of muscle under polarized light. He describes a muscular fibre as consisting of only 
two substances, which he terms respectively Hauptsubstanz and Zwischensubstanz, which 
are disposed in alternating disks. Those of the latter substance he considers to be 
represented by the dark granular line in the centre of the bright bands; all the rest is 
Hauptsubstanz. The latter, according to him, is anisotropous, the former isotropous. 
Since the granular line referred to obviously corresponds with the line formed by the 
contiguous heads of the muscle-rods, it will be seen that Heppner’s account, so far as 
the mere description goes, very nearly agrees with that given by me. 
An entire corroboration of the statements above given of the appearance of resting 
muscle under polarized light has turned up quite unexpectedly in a paper by Dr. F. 
MERKELf, which has unfortunately only been in my hands a day or two, in which he 
makes the following statement (p. 294) : — 
“ In a muscular fibre at rest, viewed under crossed Nicols, the contractile substance 
and the terminal disks appear bright and clear ; the intermediate substance on the other 
hand so delicately ( exquisit ) singly refracting, that, when the field is completely darkened, 
there is no trace of it to be seen : the doubly refracting parts lie apparently immediately 
in contact with one another.” 
Putting aside Merkel’s peculiar nomenclature of the parts which, according to his 
view, compose the substance of the muscle, it is clear that the appearance seen by him 
indicates that the whole fibre remains bright under crossed Nicols. 
Further, I may adduce statements of Prof. Brucke, whose well-known views as to the 
effect of muscular fibres upon polarized light are entirely different from those here 
* Loc. cit. 
t “ Der Contractionsvorgang im polarisirten Licht,” Schultze’s Archiv, Band is. (Jan. 1873). 
