Fig. 44.—Stomato-gastric Nerves of Cockroach. fr.g., frontal ganglion; at., antennary nerve; conn., connective; pa.g., paired ganglia; r.n., recurrent nerve; v.g., ventricular ganglion.
In the Cockroach the stomato-gastric nerves found in so many of the higher Invertebrates are conspicuously developed. From the front of each œsophageal connective, a nerve passes forwards upon the œsophagus, outside the chitinous crura of the tentorium. Each nerve sends a branch downwards to the labrum, and the remaining fibres, collected into two bundles, join above the œsophagus to form a triangular enlargement, the frontal ganglion. From this ganglion a recurrent nerve passes backwards through the œsophageal ring, and ends on the dorsal surface of the crop (·3 inch from the ring), in a triangular ganglion, from which a nerve is given off outwards and backwards on either side. Each nerve bifurcates, and then breaks up into branches which are distributed to the crop and gizzard.[106] Just behind the œsophageal ring, the recurrent nerve forms a plexus with a pair of nerves which proceed from the back of the brain. Each nerve forms two ganglia, one behind the other, and each ganglion sends a branch inwards to join the recurrent nerve. Fine branches proceed from the paired nerves of the œsophageal plexus to the salivary glands.
The stomato-gastric nerves differ a good deal in different insects; Brandt[107] considers that the paired and unpaired nerves are complementary to each other, the one being more elaborate, according as the other is less developed. A similar system is found in Mollusca, Crustacea, and some Vermes (e.g., Nemerteans). When highly developed, it contains unpaired ganglia and nerves, but may be represented only by an indefinite plexus (earthworm). It always joins the œsophageal ring, and sends branches to the œsophagus and fore-part of the alimentary canal. The system has been identified with the sympathetic, and also with the vagus of Vertebrates, but such correlations are hazardous; the first, indeed, may be considered as disproved.
Internal Structure of Brain.
Fig. 45.—A, lobes of the brain of the Cockroach, seen from within; c, cauliculus; p, peduncle; t, trabecula. B, ditto, from the front; ocx, outer calyx; icx, inner calyx. C, ditto, from above. Copied from E. T. Newton.
The minute structure of the brain has been investigated by Leydig, Dietl, Flögel, and others, and exhibits an unexpected complexity. It is as yet impossible to reduce the many curious details which have been described to a completely intelligible account. The physiological significance, and the homologies of many parts are as yet altogether obscure. The comparative study of new types will, however, in time, bridge over the wide interval between the Insect-brain and the more familiar Vertebrate-brain, which is partially illuminated by physiological experiment. Mr. E. T. Newton has published a clear and useful description[108] of the internal and external structure of the brain of the Cockroach, which incorporates what had previously been ascertained with the results of his own investigations. He has also described[109] an ingenious method of combining a number of successive sections into a dissected model of the brain. Having had the advantage of comparing the model with the original sections, we offer a short abstract of Mr. Newton’s memoir as the best introduction to the subject. He describes the central framework of the Cockroach brain as consisting of two solid and largely fibrous trabeculæ, which lie side by side along the base of the brain, becoming smaller at their hinder ends; they meet in the middle line, but apparently without fusion or exchange of their fibres. Each trabecula is continued upwards by two fibrous columns, the cauliculus in front, and the peduncle behind; the latter carries a pair of cellular disks, the calices (the cauliculus, though closely applied to the calices, is not connected with them); these disks resemble two soft cakes pressed together above, and bent one inwards, and the other outwards below. The peduncle divides above, and each branch joins one of the calices of the same hemisphere.
This central framework is invested by cortical ganglionic cells, which possess distinct nuclei and nucleoli. A special cellular mass forms a cap to each pair of calices, and this consists of smaller cells without nucleoli. Above the meeting-place of the trabeculæ is a peculiar laminated mass, the central body, which consists of a network of fibres continuous with the neighbouring ganglionic cells, and enclosing a granular substance. The antennary lobes consist of a network of fine fibres enclosing ganglion cells, and surrounded by a layer of the same. It is remarkable that no fibrous communications can be made out between the calices and the cauliculi, or between the trabeculæ and the œsophageal connectives.