=The antennal or olfactory lobes (Deutocerebrum).=—This portion of the brain consists of two hemispherical lobes, highly differentiated for special sensorial perceptions, and connected by a slightly differentiated medullary mass, the dorsal lobe (Figs. 248, 249 _lo_), from which arise the motor fibres and those of general sensibility. The antennal lobes are in part attached to the optic ganglia, and partly to the stalked body on the same side, by the optic olfactory chiasma (Fig. 250 _fch_, _choo_), a system of fibres partially intercrossed on the median line.
=The œsophageal lobes (Tritocerebrum)= (Figs. 249, 250).—From this region the labrum and viscera are innervated, the nerves to the latter being called the visceral, sympathetic, or stomatogastric system. As Viallanes remarks, though plainly situated in front of the mouth, they are in fact post-œsophageal centres. The two lobes are situated far apart, and are connected by a bundle of fibres passing behind the œsophagus, called the transverse commissure of the œsophageal ring (Lienard). The œsophageal ganglia, besides giving rise to the labral nerves, also give origin to the root of the frontal ganglion.
The brain and other ganglia are composed of two kinds of tissue.
1. The outer slightly darker, usually pale grayish white portion consists of cortical or ganglion-cells differing in size. This portion is stained red by carmine, the cells composing it readily taking the stain.
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:—