MUSCLE. 
partly in the longitudinal dark lines, some- 
times being crosswise on the exterior, more or 
less lengthwise within. These cracks are often 
short, even, well defined; at other times the 
parts near them are much stretched, or quite 
disorganized, — differences depending on the 
brittleness or toughness of the particular fibre, 
which qualities vary very much in different 
specimens, according to the state of nutrition, 
period of examination, and other circumstances. 
Hence it is clear that the discs and fibrille 
consist of the same parts, and merely result 
from the different direction in which the mass 
breaks up. To detach a fibrilla entire is to 
remove a particle from every disc, and to take 
away a disc is to abstract a particle of every 
fibrilla. Thus, every disc consists of a 
particle of every fibrilla, and every fibrilla 
of a particle of every disc. Therefore every 
fibrilla of the same fibre has the same number 
of particles, and every disc in like manner is 
composed of the same number of particles. 
If, now, isolated discs and fibrillz be examined 
under a high magnifying power, they will be 
found to bear out, in the fullest manner, the 
description that has been given. The discs are 
marked on the edge by the fragments of the 
longitudinal lines, and if regarded on their flat 
Surface, present a finely granular appearance, 
the granules being equal in diameter with the 
fibrille (fig. 288). In fact, the dark lines be- 
i Surface of a dise separated from an ele- 
Pe: ith coadiod JSibre fe timed which had 
lain long ao iapirtin It Hs aap 
ly granular structure spoken of in 
is The granules pam intended 
to be represented equal in size. Mag- 
nified 300 
diam, 
_ tween the granules are the fragments of the 
longitudinal lines of the interior of the fibre. 
_ Again, the fibrille, whether taken from the 
surface or from the interior, are always found 
to be marked at intervals by transverse dark 
lines, which are nothing more than the frag- 
_ ments of the transverse lines seen on and in the 
' fibre. They uniformly correspond with them in 
. distance and force (fig. 287, c). Thus, whether 
_ the fibre cleave crosswise or lengthwise, the 
_ resulting fragments bear in their structure their 
_ respective portions of the lines, taking an op- 
_ posite course, and evincing a co-existent ar- 
rangement in the opposite direction; and when 
a detached disc or fibrilla is itself broken, the 
fracture follows the lines thus imprinted in its 
_ Structure. 
It remains to inquire, what is the nature 
and meaning of the dark lines so often men- 
tioned ? 
They can be best examined in the separated 
dises or fibrille; and they appear to be un- 
doubtedly the results of an unequal refraction 
of the light transmitted through the object. 
The light spaces intercepted between them, 
and which by their union constitute the discs 
and fibrille, have the aspect of small lenses or 
_ particles of higher refractive power than the 
_ connecting material, which consequently is in 
_darkness when the inclosed spaces are in focus. 
By placing the object out of focus, however, 
a 
, 
A 
“3 
q 
509 
the light and dark parts are reversed, which is 
precisely what occurs with true lenses. I have 
had a series of beaded rods of glass con- 
structed, which have exactly the same ap- 
pearance as the fibrille; and when two of these 
are regarded between the observer and the 
window, one being in front of the other, and 
their beads corresponding, the dark circum- 
ferences, visible round the beads of each rod 
when seen separately, are found to be converted 
into transverse bars, crossing the rods at right 
angles in the interval of the beads; or, in other 
words, forming the elements of the transverse 
stripes. 
My friend, Dr. Gruby, of Vienna, informed 
me that he had had spiral rods of glass con- 
structed, which, when placed in front of one 
another, have the same appearance as that often 
met with in the fibres, and he conceives the 
fibrill to be, consequently, spiral threads: an 
opinion advanced by Muys, to explain the 
phenomenon of contraction, but unnecessary 
for that purpose, and which is quite at variance 
with all I have observed on the subject. Such 
spiral rods, however apposed, can never pre- 
sent lines absolutely transverse, such as always 
exist on the unmutilated fibre, and generally 
on the detached fibrille ; and the minute zig- 
gags the stripes so often form, and which might, 
if constant, be possibly explained by the 
notion of spiral rods, are the mere result of a 
stretching and disturbance of the direction of 
the axes of the particles composing the discs 
and fibrille. But the cleavage of the fibre 
into discs is especially opposed to the idea of a 
spiral form of each fibrilla. 
I think it is clear that the dark lines in both 
directions are not occasioned by a difference 
of colour, but solely by a variety in refraction ; 
but on what this difference in refraction de- 
pends it is more difficult to explain. Is the 
connecting material of a different refractive 
power, or of the same nature as the particles it 
unites? If of the same nature, it must be of 
smaller dimensions, and minute interspaces 
must be left; but of the existence of such in- 
terspaces there is no conclusive evidence. It 
seems more probable that the connecting ma- 
terial is less dense, and fills up every interval ; 
but I do not pretend to determine what may 
be its nature, or whether it differs chemically 
from the parts it serves to join. 
It is remarkable that the direction of the 
cieavage should vary so much in different spe- 
cimens, without it being possible to say on 
what the variety depends: and the question 
has still to be determined, whether the trans- 
verse and longitudinal modes of union between 
the particles are the same. It is most likely 
that they are, and the differences in the regu- 
larity and breadth of the transverse and longi- 
tudinal lines are easily explained on that sup- 
position. 
The transverse dark intervals between the 
particles, being all ranged on the same plane, 
the edge of which is directed to the observer, 
when he looks on the side of a fibre, appear 
as a sharp line, while the longitudinal dark 
intervals not being on a plane, are seen irre- 
