ANATOMY OF THE NERVE CENIRES. 471 
end, and, after traversing a thin layer of nerve cells, the sub- 
coronal ganglionic layer, they can be traced into the external 
medulla (0c), and through it into the external gray matter, 
the coronal ganglionic layer (g?), where they converge and 
cross each other, forming the chiasma of the optic nerve (07). 
3. The External Medulla, or optic cap (oc), is a large cup- 
shaped, laminated organ. It exhibits three lamella, the 
internal, intermediate, and external. 
The internal and external lamellz 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*, g*, Fig. 61). I dis- 
tinguish this lamella as the teenia, 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", ¢*) (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?). 
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*) has also been described in 
this insect by Ciccacio ; this may be termed the posterior cell 
