Thus, every action of the crayfish, which involves motion, means the contraction of one or more muscles. But what sets muscle contracting? A muscle freshly removed from the body may be made to contract in various ways, as by mechanical or chemical irritation, or by an electrical shock; but, under natural conditions, there is only one cause of muscular contraction, and that is the activity of a nerve. Every muscle is supplied with one or more nerves. These are delicate threads which, on microscopic examination, prove to be bundles of fine tubular filaments, filled with an apparently structureless substance of gelatinous consistency, the _nerve fibres_ { 102} (fig. 23). The nerve bundle which passes to a muscle breaks up into smaller bundles and, finally, into separate fibres, each of which ultimately terminates by becoming continuous with the substance of a muscular fibre (fig. 19, F). Now the peculiarity of a muscle nerve, or _motor_ nerve, as it is called, is that irritation of the nerve fibre at any part of its length, however distant from the muscle, brings about muscular contraction, just as if the muscle itself were irritated. A change is produced in the molecular condition of the nerve at the point of irritation; and this change is propagated along the nerve, until it reaches the muscle, in which it gives rise to that change in the arrangement of its molecules, the most obvious effect of which is the sudden alteration of form which we call muscular contraction.
[Illustration: FIG. 23.—_Astacus fluviatilis._—Three nerve fibres, with the connective tissue in which they are imbedded. (Magnified about 250 diameters.) _n_, nuclei.]
[Illustration: FIG. 24.—_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.]
If we follow the course of the motor nerves in a { 103} direction away from the muscles to which they are distributed, they will be found, sooner or later, to terminate in _ganglia_ (fig. 24 A, _gl.c_; fig. 25, _gn. 1–13_). A ganglion is a body which is in great measure composed of nerve fibres; but, interspersed among these, or disposed around them, there are peculiar structures, which are termed _ganglionic corpuscles_, or _nerve cells_ (fig. 24, B). These are nucleated cells, not unlike the epithelial cells which have been already mentioned, but which are larger { 105} and often give off one or more processes. These processes, under favourable circumstances, can be traced into continuity with nerve fibres.