VI] THE NERVOUS SYSTEM 129 



which exceeds in intensity the layer of retinal pigment in the eye 

 itself. 



2. The ganglionic plate, ganglionic layer, or periopticon (gp). 

 This is a kind of screen placed across the path of the post-retinal 

 fibres. It is chiefly formed of an outer layer of cylindrical columns 

 of nervous matter without nuclei (except for a nerve-cell here and 

 there), and lying more or less in line with the ommatidial axes. 

 The number of columns, however, bears no definite relation to 

 the number of ommatidia. Somewhat separated from this is an 

 inner and much narrower layer of granular nervous matter enclosing 

 numerous nuclei. The post-retinal fibres pierce both layers, and 

 fill up the space between them. Thus this second division of the 

 optic ganglion is, like the first, somewhat complex. 



3. The external chiasma (ec). After leaving the ganglionic 

 plate, the nerve-fibres from the eye cross one another very com- 

 pletely, forming a distinct chiasma. 



4. The external medullary mass, or epiopticon (em). This 

 forms a large and very conspicuous U-shaped gutter, with its 

 concavity towards the brain. It consists chiefly of a mass of 

 granular nerve-matter, which receives the nerve-fibres from the 

 external chiasma, and passes them on again to the internal chiasma. 

 Its structure is, however, complicated by the attachment of four 

 ganglionic masses, known as the ganglionic crown (gc), the corner 

 ganglion (eg), the anterior ganglionic mass (ag) and the internal 

 ganglionic mass (ig). Each of these is formed of small uni- 

 polar nerve-cells, with prolongations passing into the main 

 medullary mass. Their names sufficiently indicate their relative 

 positions. 



5. The internal chiasma (ic). The nerve-fibres from the 

 external medullary mass, on passing out, cross again completely, 

 forming a second chiasma. 



6. The internal medullary mass, or opticon (im). This is a 

 large semi-ovoid mass, whose convex surface receives the fibres 

 of the internal chiasma. It consists of three separate capsules of 

 granular nervous matter. The most external and posterior of 

 these is directly united with the external medullary mass by fibres 

 quite independent of the internal chiasma. The whole of the 

 opticon is covered with unipolar nerve-cells, and is formed from 



T. D.-F. 9 



