ANATOMY OF THE NERVE CENTRES. 47 1 



end, and, after traversing a thin layer of nerve cells, the sub- 

 coronal ganglionic layer, they can be traced into the external 

 medulla (oc), and through it into the external gray matter, 

 the coronal ganglionic layer (g' 2 ), where they converge and 

 cross each other, forming the chiasma of the optic nerve (on}. 



3. The External Medulla, or optic cap (oc), is a large cup- 

 shaped, laminated organ. It exhibits three lamellae, the 

 internal, intermediate, and external. 



The internal and external lamellae closely resemble the 

 corpus ovale and the posterior capsule, and are very dense 

 and opaque. The intermediate lamella is more transparent, 

 and consists chiefly of a sheet of fibres coursing in planes 

 parallel with the faces of the medulla. A number of these 

 leave the anterior edge of the cap and form Cuccati's bundle ; 

 the remainder terminate in the cortex (g 5 , g*, Fig. 61). I dis- 

 tinguish this lamella as the taenia, from its band-like appearance 

 in sections. 



4. The Cortex. Viallanes distinguishes several regions in the 

 cortex of the optic ganglion. That portion which surrounds 

 the internal medulla is, in his nomenclature, the wedge-shaped 

 ganglion (g l , g 4 ) (ganglion en coin) ; the part which penetrates 

 between the inner and outer medullary masses is the internal 

 ganglionic medulla ; and that on the convex surface of the 

 outer medulla the coronary ganglion (g 2 ). 



As the term ganglion applied to a region of the cortex is 

 misleading, I shall speak of these as regions of the cortex. 

 The ganglion en coin will be termed the annular region, the 

 coronary ganglion the corona, and the internal ganglionic lamella 

 the sub-corona. 



Viallanes also distinguishes a group of large nerve cells, 

 which are very distinct from the small round cells which make 

 up the rest of the cortex. These appear to be very constant in 

 different insects. He terms this group of cells the anterior 

 ganglionic mass (g*, Fig. 61). It has also been observed by 

 Ciccacio in the Blow-fly. I term it simply the anterior cell 

 group. A second similar group (g 3 ) has also been described in 

 this insect by Ciccacio ; this may be termed the posterior cell 



