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AREOLAR TISSUE, TENDONS, AND 3IUSCLES. 1H3

equal breadth, but when the fil)re is considerably extended a dotted line
becomes visible in the centre of the light band. This characteristic
cross-striped appearance is found in all voluntary muscles, but is not
absolutely confined to them, as it is seen in the fibres of the heart, which
Fig. 85.

A, portion of a medium-sized Human Muscular Fibre (magnified nearlj- 800 diameters) : B, separated
bundles of fibrils, equally magnified; f, a, large, and l>, h, smaller collections; c, still smaller; rf, the smallest whicli could be detached.
is considered an involuntary organ, though it is claimed that some per-
sons have partial control over it.
There is also a longitudinal striation seen in voluntary muscular
fibre, better marked where the transverse striation is somewhat indis-
tinct. After hardening in alcohol, voluntary muscular fibre may be
broken up longitudinally into so-called fil)rils. The fibre is not, how-
ever, composed entirely of fibrils, but contains a considerable quantity
of an intermediate substance. After the action of dilute acids or of
gastric juice on muscle, the fibres display a disposition to break up trans-
versely" in a direction parallel to the bands, and even into transverse
plates or discs formed by the lateral adhesion of the particles of approx-
imated fibrils. This separation of muscular fibre into discs is only pos-
sible after the coagulation of muscle-plasma or the action of reagents
upon it.
Muscular fibres also exhibit a number of clear oval nuclei (Fig. 86).
In the muscles of mammals these nuclei are situated upon the under
surface of the sarcolemma. Surrounding these nuclei there is sometimes
a certain amount of granular matter which is derived from the original
primitive embryonic protoplasm.
The nuclei of muscular fibre are not readily seen without the addi-
One or two nucleoli may also be found within each
tion of acetic acid.
nucleus.