472 THE NERVOUS SYSTEM, 
group. The cells of the anterior group are connected, accord- 
ing to Ciccacio, with the tenia; those of the posterior group 
appear to be the origin of some of the radiating fibres of the 
posterior capsule. 
The Internal Chiasma.—This term is applied by Viallanes to 
the radiating fibres of the middle capsule. In some insects 
these fibres form a distinct chiasma, crossing each other com- 
pletely; in others, as in the Blow-fly, there is no distinct 
chiasmatic crossing. It appears to me that when these fibres 
cross, those of the optic nerve do not, and vice versd ; at any 
rate, this is sometimes the case. I have therefore discarded 
the term internal chiasma. 
The Fibres of Cuccati pass from the tenia to the posterior 
part of the pyramidal ganglionic mass, and form a thin com- 
missure between the two optic ganglia. It appears to me 
probable that the slender bundles described by Cuccati as the 
fork, which lie behind the pyramid, are derived in part from 
these fibres, and connect the optic ganglia with the origin of 
the nerves to the simple eyes. I cannot say positively, how- 
ever, that such is the case, as I have been unable to trace the 
bundle of Cuccati, as he has done, to the region in which the 
fork lies. 
The Histological Structure of the External Medulla, or Optic Cap. 
—I know no organ which is more difficult to resolve into its 
histological element than the internal and external medullary 
substance of the optic ganglion. The structure of the corpus 
ovale and the posterior capsule is very similar to, if not identical 
with, that of the outer and inner lamellz of the optic cap, but, 
owing to the regular arrangement of the elements of the latter, 
it is more conveniently investigated. 
In good radial sections the outer and inner layers of the 
optic cap appear to consist of very closely-packed prismatic 
fibres, supported by a distinct neuroglia similar to Miiller’s 
fibres in the vertebrate retina. In some sections the prismatic 
fibres have the appearance of oblong or ovoid cells, and I 
formerly described them assuch. My more recent preparations 
indicate, I think, that my former interpretation of the appear- 
