=Delbœuf.= Nains et géants, Étude comparative de la force des petits et des grands animaux. Bruxelles. (Also in Kosmos, xiii, 1883, pp. 58–62.) =Camerano.= Mem. Acc. Torino (2), xliii, 1893, p. 229. Also Newport, Art. Insecta, p. 76. Kirby and Spence, Burmeister, Graber, Kolbe, pp. 375, 376. THE NERVOUS SYSTEM _a._ The nervous system as a whole [Illustration: FIG. 239.—Central nervous system of _Machilis maritima_: _au_, eye; _lo_, optic tract; _g_, brain; _an_, antennal nerve; _oe_, œsophagus passing between the œsophageal commissures; _usg_, infraœsophageal ganglion; I-III, thoracic ganglia; 1–8, abdominal ganglia, the last (_Sabc_) consisting of three fused ganglia; _s_, sympathetic nervous system of the ventral cord.—After Oudemans, from Lang. ] The nervous system of insects consists of a double series or chain of ganglia connected by nervous cords or commissures. The first of these is the brain or supraœsophageal ganglion; it is situated in the upper part of the head, above the gullet or œsophagus, while the rest of the system, called the ventral cord, lies on the floor of the body, under the digestive canal. A ganglion or nerve-centre consists of a mass of ganglion-cells, from each of which a process or fibre passes off, uniting with others to form a nerve; by means of these nerves the ganglia are connected with other ganglia, and with the sensory cells and muscle-fibres. The ganglia may be simple, and arranged in pairs, corresponding to each segment of the body, or they may be compound, the result of the fusion of several pairs of ganglia, which in the early stages of the embryo are separate. Thus the brain of insects is a compound ganglion, or ganglionic mass. The nerves are of two kinds: 1. _Sensory_, which transmit sensations from the peripheral sense-cells to the ganglion, or brain; 2. _Motor_, which send stimuli from the brain or any other ganglion to the muscles. Of ganglion cells, some are tactile, and others give rise to nerves of special sense, being distributed to the eyes, or to the organs of hearing, smell, taste, or touch. [Illustration: