ANATOMY OF THE NERVE CENTRES. 453 



Nerves. The following nerves arise from the brain : the 

 antennal nerves from the olfactory lobes, the pharyngeal nerves 

 (ph) from the crura, and the maxillary nerves (mx) from the 

 infra-resophageal ganglia. A single median nerve makes its 

 exit between the hemispheres of the cerebron from the upper 

 part of the organ ; it supplies the ocelli, or simple eyes. 



The term optic-nerve has been applied to the peduncles of 

 the great pyriform optic ganglia, and also to the crossed 

 fibres which arise from their external convex surface. I 

 term the former the optic peduncles, and the latter the optic 

 nerves. 



The Cerebron. Under this term the whole central mass of 

 the brain above the ossophagus is included. It consists of a 

 pair of sub-hemispherical, lobulated, concavo-convex discs of 

 reticular substance, known as the hemispheres, which enclose a 

 central cavity, containing the trabeculse, the corpus centrale, 

 the nodulus, and the procerebral lobes, as well as the pyra- 

 midal ganglion. The hemispheres consist in great part of the 

 mesocerebron, which is far more largely developed than the 

 thalamic lobes and the metacerebron in the Diptera. The 

 thalamic lobes (tJi) are so intimately united with the meso- 

 cerebron that they merely form lobes of the hemispheres, and 

 the metacerebron is entirely concealed in the posterior part of 

 the mesocerebron. It is represented by little more than a 

 commissure (PL XXXII., Fig. 3). 



The Median Fissure. The two hemispheres are separated 

 from each other above by a median fissure, which extends 

 from the posterior part of the cerebron, commencing above 

 the transverse commissure of the metacerebron, to the an- 

 tennal ganglia in front. It is wide behind and narrow in front, 

 and its anterior part is closed by the commissures of the 

 frontal lobes of the mesocerebron and of the procerebral 

 lobes. It is filled by the cortex which passes from the surface 

 into the central cavity. The nerve to the ocelli traverses it 

 above the pyramidal ganglion. 



Lobes of the Mesocerebron. Externally the mesocerebron is 

 connected with the optic peduncles (Fig. 58, o s.) Above and 



