last[102]. In the spinal marrow of the rhinoceros-beetle, both larva and imago, the nerves consist of simple filaments which diverge like rays in all directions[103]: the same circumstance distinguishes the cheese-maggot, only some of the nerves appear to branch at the end[104]: in the louse, the last ganglion sends forth posteriorly three pairs of nerves which render to the abdomen[105]. Sometimes, though rarely, nerves originate in the internodes of the spinal marrow. Cuvier indeed has asserted that in invertebrate animals all the nerves spring from the ganglions, and never immediately from the spinal marrow; but Swammerdam, in describing those of the silk-worm, mentions and figures four pairs as proceeding from the four anterior internodes, excluding the first[106]; and at the same time he gives it as his opinion, that all the nerves in insects really originate from the marrow itself, and not from the ganglions, which he asserts are of a different substance, and are inclosed in the marrow for the sake of giving it greater firmness[107]. In this opinion, however, he seems singular[108]. Those remarkable nerves described by Lyonet under the name of spinal bridle (bride épinière) also take their origin, not from the ganglions, but from a bifurcation of the spinal marrow. Of these, in the caterpillar of the goat-moth there are ten, the first issuing from the bifurcation of the internode between the fourth and fifth ganglions, and the remainder from the succeeding ones. After approaching the succeeding ganglion, these nerves form a pair of branches that diverge nearly at right angles from the bridle, and producing several lesser branches, lose themselves in the sides of the animal[109]. Besides the nerves above mentioned, two generally issue from the posterior part of the last ganglion, diverging in opposite and oblique directions: some of these render to the parts of generation; and in the silk-worm, and probably other species, the innermost pair is perforated for the passage of the vasa deferentia[110].

After duly considering this general outline of the nervous system of insects, the question will continually occur to you,—is then what you have called the brain the sensorium commune of these animals, in the same manner as it is in those with warm blood? To this query a negative must be returned. In the latter, the brain is the common centre to which, by means of the nerves and spinal marrow, all the sensations of the animal are conveyed, and in which all its perceptions terminate. The nerves and spinal marrow are merely the roads by which the sensations travel; and if their communication with the brain, by any means be cut off at the neck, the whole trunk of the animal becomes paralytic, evidently proving that the organ by which it feels is the brain. This, however, is so far from being the case in insects, that in them, if the head be cut off, the remainder of the body will continue to give proofs of life and sensation longer than the head: both portions will live after the separation, sometimes for a considerable period; but the largest will survive the longest, and will move, walk, and occasionally even fly, at first almost as actively without the head, as when united to it. Lyonet informs us, that he has seen motion in the body of a wasp three days after it had been separated from the head; and that a caterpillar even walked some days after that operation; and when touched, the headless animal made the same movements as when intire[111]. Dr. Shaw has observed—an observation confirmed in Unzer's Kleine Schriften,—that if Geophilus electricus be cut in two, the halves will live and appear vigorous even for a fortnight afterwards; and what is more remarkable, that the tail part always survives the head two or three days[112]. The sensorium commune of insects, therefore, does not, as in the warm-blooded animals, reside in the brain alone, but in the spinal marrow also. It was on this account probably that Linné denied the existence of a brain in insects, regarding it merely as the first ganglion of the spine.

Cuvier and other modern physiologists, from the ganglionic structure of this organ, are of opinion that it is not the analogue of the cerebro-spinal system of vertebrate animals, but rather of their great sympathetic nerves. Indeed, considering solely the external structure of the nervous system of insects, a great resemblance strikes us between it and these nerves; for besides its general ganglionic structure, there is also in them an upper ganglion in the neck, seemingly corresponding with what we have named the brain of insects, from which the nervous chord dips to the lower part of the neck, where it forms a second ganglion, which appears to correspond with what we have considered as their second ganglion[113]. We may observe, however, that at least in one respect there is even an external resemblance between the brain of insects and that of vertebrate animals:—it most commonly consists, as has been stated, like them, of two lobes, often very distinct; a circumstance which not unfrequently distinguishes the other ganglions[114], and is not borrowed from the ganglions of the great sympathetics. With respect