The nervous centres of the head form a thick, irregular ring, which swells above and below into ganglionic enlargements, and leaves only a small central opening, occupied by the œsophagus. The tentorium separates the brain or supra-œsophageal ganglion from the sub-œsophageal, while the connectives traverse its central plate. Since the œsophagus passes above the plate, the investing nervous ring also lies almost wholly above the tentorium. [Illustration: Fig. 40.--Side view of Brain of Cockroach, × 25. _op_, optic nerve; _oe_, œsophagus; _t_, tentorium; _sb_, sub-œsophageal ganglion; _mn_, _mx_, _mx_′, nerves to mandible and maxillæ. Copied from E. T. Newton.] The brain is small in comparison with the whole head; it consists of two rounded lateral masses or hemispheres, incompletely divided by a deep and narrow median fissure. Large optic nerves are given off laterally from the upper part of each hemisphere; lower down, and on the front of the brain, are the two gently rounded antennary lobes, from each of which proceeds an antennary nerve; while from the front and upper part of each hemisphere a small nerve passes to the so-called “ocellus,” a transparent spot lying internal to the antennary socket on each side in the suture between the clypeus and the epicranium. The sub-œsophageal ganglion gives off branches to the mandibles, maxillæ, and labrum. While, therefore, the supra-œsophageal is largely sensory, the sub-œsophageal ganglion is the masticatory centre. The œsophageal ring is double below, being completed by the connectives and the sub-œsophageal ganglion; also by a smaller transverse commissure, which unites the connectives, and applies itself closely to the under-surface of the œsophagus.[101] [101] This commissure, which has been erroneously regarded as characteristic of Crustacea, was found by Lyonnet in the larva of Cossus, by Straus-Dürckheim in Locusta and Buprestis, by Blanchard in Dytiscus and Otiorhynchus, by Leydig in Glomeris and Telephorus, by Dietl in Gryllotalpa, and by Liénard in a large number of other Insects and Myriapods, including Periplaneta. See Liénard, “Const. de l’anneau œsophagien,” Bull. Acad. Boy. de Belgique, 2^e Sér., Tom. XLIX., 1880. Two long connectives issue from the top of the sub-œsophageal ganglion, and pass between the tentorium and the submentum on their way to the neck and thorax. The three thoracic ganglia are large (in correspondence with the important appendages of this part of the body) and united by double connectives. The six abdominal ganglia have also double connectives, which are bent in the male, as if to avoid stretching during forcible elongation of the abdomen. The sixth abdominal ganglion is larger than the rest, and is no doubt a complex, representing several coalesced posterior ganglia; it supplies large branches to the reproductive organs, rectum, and cerci. _Internal Structure of Ganglia._