[Illustration: FIG. 53.—_Astacus fluviatilis._—A, living muscular fibres very highly magnified; _B_, a fibrilla treated with solution of sodium chloride; _C_, a fibrilla treated with strong nitric acid, _s_, septal lines; _sz_, septal zones; _is_, interseptal zones; _a_, transverse line in the interseptal zone.] {183} This much is readily seen in a specimen of muscular fibre taken from any part of the body, and whether alive or dead. But the results of the ultimate optical analysis of these appearances, and the conclusions respecting the normal structure of striped muscle which may be legitimately drawn from them, have been the subjects of much controversy. Quiescent muscular fibres from the chela of the forceps of a crayfish, examined while still living, without the addition of any extraneous fluid, and with magnifying powers of not less than seven or eight hundred diameters, exhibit the following appearance. At intervals of about 1‐4000th of an inch, very delicate but dark and well-defined transverse lines are visible; and these, on careful focussing, appear beaded, as if they were made of a series of close-set minute granules not more than 1‐20,000th to 1‐30,000th of an inch in diameter. These may be termed the _septal lines_ (fig. 52, D and E, _a_; C, _1–5_; fig. 53, _s_). On each side of every septal line there is a very narrow perfectly transparent band, which may be distinguished as the _septal zone_ (fig. 53, _sz_). Upon this follows a relatively broad band of a substance which has a semi-transparent aspect, like very finely ground glass, and hence appears somewhat dark relatively to the septal zone. Upon this _inter-septal zone_ (_i s_) follows another septal zone, then a septal line, another septal zone, an inter-septal zone, and so on throughout the whole length of the fibre. {184}