472 THE NERl'OUS 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 versa ; at any 

 rate, this is sometimes the case. I have therefore discarded 

 the term internal chiasma. 



The Fibres of Cuccati pass from the t?enia 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 lamellae 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 neurogha similar to Muller'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 as such. My more recent preparations 

 iinlirate, I think, that my former interpretation of the appear- 



