In living and recently dead muscle, as well as in muscles which have been preserved in spirit or hardened with nitric acid, the inter-septal zones polarize light; and hence, in the dark field of the polarizing microscope, the fibre appears crossed by bright bands, which correspond with the inter-septal zones, or at any rate, with the middle parts of them. The substance which forms the septal zones, on the contrary, produces no such effect, and consequently remains dark; while the septal lines again have the same property as the inter-septal substance, though in a less degree.
In fibres which have been acted upon by solution of salt, or dilute acids, the inter-septal zones have lost their polarizing property. As we know that the reagents in question dissolve the peculiar constituent of muscle, myosin, it is to be concluded that the inter-septal substance is chiefly composed of myosin.
Thus a fibril may be considered to be made up of {187} segments of different material arranged in regular order; S–sz–IS–sz–S–sz–IS–sz–S: S representing the septal line; sz, the septal zone; IS, the inter-septal zone. Of these, IS is the chief if not the only seat of the myosin; what the composition of sz and of S may be is uncertain, but the supposition, that, in the living muscle, sz is a mere fluid, appears to me to be wholly inadmissible.
When living muscle contracts, the inter-septal zones become shorter and wider and their margins darker, while the septal zones and the septal lines tend to become effaced—as it appears to me simply in consequence of the approximation of the lateral margins of the inter-septal zones. It is probable that the substance of the intermediate zone is the chief, if not the only, seat of the activity of the muscle during contraction.
5. The elements of the nervous tissue are of two kinds, nerve-cells, and nerve fibres; the former are found in the ganglia, and they vary very much in size (fig. [54], B). Each ganglionic corpuscle consists of a cell body produced into one or more processes which sometimes, if not always, end in nerve fibres. A large, clear spherical nucleus is seen in the interior of the nerve-cell; and in the centre of this is a well defined, small round particle, the nucleolus. The corpuscle, when isolated, is often surrounded by a sort of sheath of small nucleated cells. {188}
FIG. 54.—Astacus fluviatilis.—A, one of the (double) abdominal ganglia, with the nerves connected with it (× 25); B, a nerve cell or ganglionic corpuscle (× 250). a, sheath of the nerves; c, sheath of the ganglion; co, co′, commissural cords connecting the ganglia with those in front, and those behind them. gl.c, points to the ganglionic corpuscles of the ganglia; n, nerve fibres.
The nerve fibres (fig. [55]) of the crayfish are remarkable for the large size which some of them attain. In the central nervous system a few reach as much as 1‐200th of an inch in diameter; and fibres of 1‐300th or 1‐400th of an inch in diameter are not rare in the main branches. Each fibre is a tube, formed of a strong and elastic, sometimes fibrillated, sheath, in which nuclei are imbedded at irregular intervals; and, when the nerve trunk gives {189} off a branch, more or fewer of these tubes divide, sending off a prolongation into each branch.
FIG. 55.—Astacus fluviatilis.—Three nerve fibres, with the connective tissue in which they are imbedded (magnified about 250 diameters); n, nuclei.