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} [Illustration: 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. [Illustration: FIG. 55.—_Astacus fluviatilis._—Three nerve fibres, with the connective tissue in which they are imbedded (magnified about 250 diameters); _n_, nuclei.] When quite fresh, the contents of the tubes are perfectly pellucid, and without the least indication of structure; and, from the manner in which the contents exude from the cut ends of the tubes, it is evident that they consist of a fluid of gelatinous consistency. As the fibre dies, and under the influence of water and of many chemical re-agents, the contents break up into globules or become turbid and finely granular. Where motor nerve fibres terminate in the muscles to which they are distributed, the sheath of each fibre becomes continuous with the sarcolemma of the muscle, and the subjacent protoplasm is commonly raised into a small prominence which contains several nuclei (fig. 52, F). These are called the _terminal_ or _motor plates_. {190} 6, 7. The _ova_ and the _spermatozoa_ have already been described (pp. 132–135). * * * * *