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TEXT-BOOK OF ENTOMOLOGY 



of minute fibrillae, the neurospongium of Hickson. In the young 

 cockroach (Periplaneta) the optic nerve separating the cerebral 

 ganglion from the opticon is much longer in proportion than it is in 

 the adult blow-fly. 



The second ganglionic swelling (epiopticon, Fig. 261, c.op) is 

 separated from the opticon by a tract of fine nerve-fibrils, which 

 partially decussate; at the decussation two or three larger nerve- 

 cells may be seen. It also contains a few scattered nerve-cells (n.c). 



The third ganglionic swelling (peri- 

 opticon, p.op) is separated from the 

 others by a bundle of long optic 

 nerve-fibrils, which cross one another. 

 It is composed of a number of cylin- 

 drical masses of neurospongium 

 arranged side by side (Fig. 261, 

 p.op). Between these elements of 

 the periopticon, which do not seem 

 to bear any relation to the number 

 of ommatidia, a single nerve-cell is 

 very frequently seen. The periopti- 

 con does not occur in Periplaneta and 

 Nepa (Hickson). The three optic 

 ganglia thus described, together with 

 the cerebral ganglia, are surrounded 

 by a sheath of densely packed nerve- 

 cells. 



FIG. 266. Periopticon and terminal 

 anastomosis of Agrion, showing the char- 

 acter of the elements of the periopticon 

 (p.op) and the structure of the terminal 

 anastomosis (t.a). I. The first layer of the 

 terminal anastomosis, consisting of a plexus 

 of fibrils and nerve-cells (n.c). 2. The sec- 

 ond layer, in which the fibrils are collected 

 together in bundles. 3. The final optic 

 plexus and nerve-cells. 4. The layer in 

 which the optic fibrils are collected in bun- 



Bearing in mind the fact that the re- 

 tinulse are the nerve-end cells of the fibres 

 passing through the periopticon, it will be 

 well to read the following account, by Hick- 

 son, of the terminal anastomosis of the 



Optic fibrils in the periopticon of Ai/rinit 



Afi*' Wurcatum, and to examine his sketch 

 (Fig. 266): 



" The terminal anastomosis of Agrion may be conveniently divided into four 

 regions. First the region (1) lying nearest to the periopticon in which the 

 nerve-cells are numerous, and the fibrils leaving the periopticon form a compli- 

 cated plexus; the region (2) next to this, in which the fibrils have collected into 

 bundles separated by spaces occupied by very thin-walled trachea- in which 

 there are no spiral markings, and lymph-spaces; next, the region (3) in which 

 the fibrils form a final plexus, and in which there are again a considerable num- 

 ber of nerve-cells ; and, lastly, the region (4) in which the fibrils arc again col- 

 lected into bundles, separated by spaces containing tracheae, which perforate the 

 basement membrane to supply the retinulse." 



It would seem as if the decussation of the optic nerve-fibrils were a matter of 



