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 (1) from the 
infra-cesophageal 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. 1 
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 cesophagus 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 trabecule, 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 (i) 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, 0 s.) Above and 
