The large ganglion cells (represented in Figs. 252 and 253) are oval, and send off usually a single nerve-fibre; they have a thin fibrous cell-wall, and the contents are finely granular. The nucleus is very large, often one-half the diameter of the entire cell, and is composed of large round refractive granules, usually concealing the nucleolus. 2. The medullary or inner part of the brain consists of matter which remains white or unstained after the preparation has remained thoroughly exposed to the action of the carmine. It consists of minute granules and interlacing fibres. The latter often forms a fine irregular network inclosing masses of finely granulated nerve matter. This is called by Dietl “marksubstanz.” Leydig, in his Vom Bau des thierischen Körpers, p. 89, thus refers to it:— “In the brain and ventral ganglia of the leech, of insects, and in the brain of the gastropods (Schnecken) I observe that the stalks (stiele) of the ganglion-cells in nowise immediately arise as nerve-fibres, but are planted in a molecular mass or _punktsubstanz_, situated in the centre of the ganglion, and merged with this substance. It follows, from what I have seen, that there is no doubt that _the origin of the nerve-fibres first takes place from this central punktsubstanz_.” “This relation is the rule. But there also occur in the nerve-centres of the invertebrates single, definitely situated ganglion-cells, whose continuations become nerve-fibres without the intervention of a superadded _punktsubstanz_.” We may, with Kenyon, call it the fibrillar substance. Leydig subsequently (p. 91) further describes this fibrillar substance, stating that the granules composing it form a reticulated mass of fibrillæ, or, in other words, a tangled web of very fine fibres:— “We at present consider that by the passage of the continuation of the ganglion-cells into the _punktsubstanz_ this continuation becomes lost in the fine threads, and on the other side of the _punktsubstanz_ the similar fibrillar substance forms the origin of the axis-cylinders arranged parallel to one another; so it is quite certain _that the single axis-cylinder derives its fibrillar substance as a mixture from the most diverse ganglion-cells_.” _d._ The visceral (sympathetic or stomatogastric) system This system in insects is composed (1) of a series of three unpaired ganglia (Fig. 249, _gv^1_, _gv^2_, _gv^3_), situated over the dorso-median line of the œsophagus, and connected by a median nervous cord or recurrent nerve (_nr_, _vagus_ of Newport). The first of these ganglia is the frontal ganglion, which is connected with the œsophageal ganglia by a pair of roots (_rvt_), which have an origin primitively common with that of the labral nerves (Fig. 248, _fg_ and _lbr_). [Illustration: