FIG. 23.—Astacus fluviatilis.—Three nerve fibres, with the connective tissue in which they are imbedded. (Magnified about 250 diameters.) n, nuclei.
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.
FIG. 25.—Astacus fluviatilis.—The central nervous system seen from above (nat. size). a, vent; an, antennary nerve; a′n, antennulary nerve; c, circumœsophageal commissures; gn. 1, supraœsophageal ganglion; gn. 2, infraœsophageal ganglion; gn. 6, fifth thoracic ganglion; gn. 7, last thoracic ganglion; gn. 13, last abdominal ganglion; œs, œsophagus in cross section; on, optic nerve; sa, sternal artery in cross section; sgn, stomatogastric nerve.
The chief ganglia of the crayfish are disposed in a longitudinal series in the middle line of the ventral aspect of the body close to the integument (fig. [25]). In the abdomen, for example, six ganglionic masses are readily observed, one lying over the sternum of each somite, connected by longitudinal bands of nerve fibres, and giving off branches to the muscles. On careful examination, the longitudinal connecting bands, or commissures (fig. [24], co), are seen to be double, and each mass appears slightly bilobed. In the thorax, there are six, larger, double ganglionic masses, likewise connected by double commissures; and the most anterior of these, which is the largest (fig. [25], gn. 2), is marked at the sides by notches, as if it were made up of several pairs of ganglia, run together into one continuous whole. In front of this, two commissures (c) pass forwards, separating widely, to give room for the gullet (œs), which passes between them; while in front of the gullet, just behind the eyes, they unite with a transversely elongated mass of ganglionic substance (gn. 1), termed the brain, or cerebral ganglion.
All the motor nerves, as has been said, are traceable, directly or indirectly, to one or other of these thirteen sets of ganglia; but other nerves are given off from the ganglia, which cannot be followed into any muscle. In {106} fact, these nerves go either to the integument or to the organs of sense, and they are termed sensory nerves.
When a muscle is connected by its motor nerve with a ganglion, irritation of that ganglion will bring about the contraction of the muscle, as well as if the motor nerve itself were irritated. Not only so; but if a sensory nerve, which is in connexion with the ganglion, is irritated, the same effect is produced; moreover, the sensory nerve itself need not be excited, but the same result will take place, if the organ to which it is distributed is stimulated. Thus the nervous system is fundamentally an apparatus by which two separate, and it may be distant, parts of the body, are brought into relation with one another; and this relation is of such a nature, that a change of state arising in the one part is followed by the propagation of changes along the sensory nerve to the ganglion, and from the ganglion to the other part; where, if that part happens to be muscle, it produces contraction. If one end of a rod of wood, twenty feet long, is applied to a sounding-board, the sound of a tuning-fork held against the opposite extremity will be very plainly heard. Nothing can be seen to happen in the wood, and yet its molecules are certainly set vibrating, at the same rate as the tuning-fork vibrates; and when, after travelling rapidly along the wood, these vibrations affect the sounding-board, they give rise to vibrations of the molecules of the air, which reaching the ear, are converted into an audible note. So in the nerve tract: {107} no apparent change is effected in it by the irritation at one end; but the rate at which the molecular change produced travels can be measured; and, when it reaches the muscle, its effect becomes visible in the change of form of the muscle. The molecular change would take place just as much if there were no muscle connected with the nerve, but it would be no more apparent to ordinary observation than the sound of the tuning-fork is audible in the absence of the sounding-board.