238 TEXT-BOOK OF ENTOMOLOGY 



rise to the labral nerves, also give origin to the root of the frontal 

 ganglion. 



c. Histological elements of the brain 



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 "rnarksubstanz." Leydig, in his Vom Bau des thier- 

 ischen Korpers, 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-Jibres 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.' 1 '' 

 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 fibrillse, 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 

 uiijft arc 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. 240, gv l , yv 2 , gv 3 ), situated over the dorse-median line of 

 the oesophagus, and connected by a median nervous cord or recurrent 



