(855). In the nervous system of the Insecta, we have many interesting illustrations of that gradual concentration of the parts composing it, and consequently of increased proportionate development of the nervous centres, corresponding with the more active movements and higher faculties by which the class before us is so remarkably distinguished from those forms of articulated animals that we have hitherto had an opportunity of examining. The supra-cesophageal ganglion, or brain, assumes a preponderance of size in relation to more perfect organs of sense and to instincts of more exalted character; the chain of ganglia placed along the floor of the abdomen is composed of a few large masses of sufficient power to animate the strong and energetic muscles of the limbs; and, moreover, anatomists have detected the existence of an additional nervous apparatus, apparently representing the sympathetic system of vertebrate animals, which is distributed to the viscera appropriated to digestion. Each of these divisions will therefore require a separate notice.

(856). The brain, or encephalic ganglion (fig. 164, l), is a nervous mass of considerable size placed above the gullet: it consists essentially of two ganglia united into one mass; and from it all the nerves appropriated to the special instruments of the senses are derived; so that it may naturally be regarded as the chief seat of sensation and intelligence. The nerves originating from this common sensorium are seen upon an enlarged scale in fig. 165: they are the optic (fig. 165, a), supplying the eyes, and the antennal (fig. 165, e), which run to the special instruments of touch, or antennae - organs of a very singular character, that we shall examine more minutely hereafter. Two other cords of variable length (fig. 165, g g) are given off from the inferior aspect of the brain, and serve to connect it with the anterior ganglion of the ventral chain (fig. 165, h), to which some writers have thought proper to give the name of cerebellum, though upon what grounds it is difficult to conjecture: the mass last mentioned gives off various nerves to supply the parts connected with the mandibles, maxilla?, and other organs of the mouth.

(857). The rest of the ventral chain of ganglia forms a continuous series (fig. 164, 2,3, 4, 5,6, 7, 8) of nervous centres arranged in pairs and united to each other by double cords of communication; but they vary much in number and relative magnitude in different families. Those situated in the thorax are usually of the greatest proportionate size, inasmuch as they furnish the nerves that supply the muscles of the wings and legs; the succeeding ganglia give branches to the abdominal segments; and the last, which is commonly of considerable bulk, supplies the sexual organs and the extremity of the colon.

Anatomy of Melon: a, the stomach; b, hepatic vessels; c, intestine.

Fig. 164. Anatomy of Melon: a, the stomach; b, hepatic vessels; c, intestine; d d, ovaria; e, section of ovary showing the internal cavity; f, vagina; g, spermatheca; h, i, gluten-secretors: 1, supra-oesophageal ganglion of the nervous system; 2,3,4,5, 6,7, 8, ventral ganglia; 9 9, nervus vagus; 10, cephalic nerves; 11, optic ganglion.

(858). It is the general opinion of modern physiologists, that the intimate composition of the nervous apparatus described above is by no means so simple as it appears to ordinary observation; and, since the experiments of Sir Charles Bell and Magendie demonstrated the existence of distinct columns or tracts in the spinal axis of vertebrate animals, various anatomists have endeavoured to show that corresponding parts may be pointed out in the ventral chain of articulated animals. There can, indeed, be no doubt that this portion of the nervous system of an insect corresponds in function with the medulla spinalis; and if, in the one case, the nerves which preside over the general muscular movements arise from a different column to that whence the nerves that correspond with the periphery of the body originate, while those which regulate the motions of respiration emanate from a distinct tract, we might reasonably suppose a similar arrangement to exist in the structure of the nervous system we are now examining.

It has, in fact, been well ascertained that the nerves given off to the muscular system of the Homogangliata are not derived from the ganglionic masses themselves, but from the cords which connect them together, while the nerves distributed to the integument and external parts of the body communicate immediately with the ganglia. These different modes of origin give presumptive evidence that at least two distinct tracts exist in the central axis of insects; but, from the extreme minuteness of the different parts, it is not easy satisfactorily to demonstrate them separately. In the larger Articulata, however, as for example in the Crustaceans, two distinct columns of nervous matter are readily detected; it will therefore be more convenient to defer the investigation of this interesting subject until we have an opportunity of describing these parts upon an enlarged scale; enough has been said at present to enable the reader to compare the nervous axis of an insect with that of a lobster, and draw correct conclusions from the comparison.

(859). The last division of the nervous apparatus, which we have already mentioned as being the representative of the sympathetic system, consists of two portions, - one corresponding, in distribution at least, with the nervus vagus of Vertebrata, while the other represents, apparently, the sympathetic ganglia. The nervus vagus, as we shall call it, and which has been named by Swammerdam * and Cuvier the recurrent nerve, arises (fig. 165, b b) by two roots from the opposite extremities of the brain, close to the origins of the antennal nerves. The nervous cords thus derived soon unite to form a minute central ganglion (fig. 165, i), from which proceeds a single nerve (fig. 165, f k) that runs with the gullet beneath the brain, and spreads in delicate ramifications upon the oesophagus as far as the muscular stomach (fig. 164, 9 9), or to the gizzard when that organ exists.