ANATOMY OF THE NERVE CENTRES. 469 



used, to a far greater extent than any of the other nerve- 

 elements. 



The Antennal Nerve arises partly from the glomeruli and 

 partly from the adjacent cortical substance in which its fibres 

 can be traced into large stellate nerve cells. In the Cricket 

 and man)' other insects two pairs of antennal nerves have 

 been described the large sensory nerve and a smaller motor- 

 nerve, which supplies the muscles of the antenna. They are, 

 perhaps, united into a single trunk in the Blow-fly. Possibly 

 the nerve-fibres, which arise directly from the large stellate 

 cells, are motor in function. 



The Optic Ganglion. This term has been applied with different 

 limitations. Viallanes includes under it all the nervous struc- 

 tures between the inner extremities of the great rods of the 

 compound eye and the hemispheres of the brain, but Berger 

 [178], Ciccacio,* Bellonci, and Cuccati, as I think, correctly 

 limit the use of the term to those parts included under it in the 

 following description, naming the remaining structures the 

 retina ; whilst Hicksont proposes to call the whole the retina. 



The optic ganglia of the Blow-fly differ but slightly from the 

 same structures in the Insecta generally ; these are very similar 

 to the corresponding parts in the Decapod Crustacea. 



I shall distinguish and describe the following: (i) The optic 

 peduncle ; (2) the inner medulla ; (3) the outer medulla ; and 

 (4) the cortex. The relations of these parts will be most easily 

 understood by a reference to Fig. 61. The remaining struc- 

 tures, included by Viallanes as a portion of the optic ganglion, 

 are my optic nerve and retina. 



i. The Optic Peduncle is frequently termed the optic nerve. 

 If the term optic nerve is to be used in the Arthropoda, it 

 certainly is inappropriate for the designation of this structure, 

 which unites the optic ganglion and the cerebron. In order to 

 be consistent, if the optic peduncle is to be termed the optic 

 nerve, the optic ganglion should be termed the retina, as 

 Hickson has done. To do so appears to me to be an un- 



* Mem. d. Accad. d. Sc. Bologna, torn, vi., 1884. 



f Quart. Journ. Microsc. Sr., vol. xxv., 1885. p Z- ' 5 



