THE CRREBRU.M. 273 



generally forming two ganglia. This cord sends off a branch on the 

 opposite sides to each eye, which is the optic nerve. Its entire cir- 

 cumference is covered by a thin transparent membrane, which loosely 

 surrounds it, and which in many cases, as for example, in Dylicus, 

 is beset with small darker knots, placed in regular squares (PL 

 XXXI. f. 1). The large muscles of the upper jaw spread above 

 it, extending upwards to the skull, so that it is entirely covered by 

 soft parts. The genei'al form of the brain varies in as far as the two 

 hemispheres are more or less separated. In the Coleoptera they 

 approach closely together, and indeed so closely that they form but one 

 stripe, which is merely swollen on each side near the middle ; in other 

 instances, as for example in Gryllus migratorius, the two hemispheres 

 are nearly entirely separated, and are attached together by a central 

 thin nervous cord only, analogous to the corpus callosum of the superior 

 animals. The nerves which pass from the cerebrum are: 



1. The nerve of the antennae (nervus antennatis). It originates 

 from the anterior margin of each hemisphere, but more exteriorly 

 when the antennae are lateral, and centrically when those organs are 

 inserted in the face. It runs as a simple undivided filament, which in 

 the first case passes over the tendon of the mandibles, and in the last 

 proceeds contiguously to the great flexor of the mandibles, to the root 

 of the antennae, immediately beneath the membrane which connects it 

 with the clypeus, but yet without sending off branches. In many 

 cases it is equally thick throughout, in others, for example in the bees 

 and the cockchafer, it is more or less swollen at its base. When 

 arrived at the antennae the main stem still runs in this direction, and 

 very distinctly to the apex of the organ, and between the muscles, but 

 it gives off on all sides delicate auxiliary branches to the muscles them- 

 selves. It is accompanied by a single branch of the trachea, which 

 originates on each side from the superior stem of the head, running 

 between the flexors of the mandibles, and branching off according to the 

 ramifications of the nerve itself. 



2. The optic nerve which originates from the lateral margin of each 

 hemisphere, with either a thicker or a thinner base, and extends to the 

 orbit, becoming gradually clavate. It varies much in form, but it always 

 retains the general characteristic of gradually distending. In Dylicus 

 it originates with a thin base, then suddenly distends, and afterwards 

 runs as a straight cylinder to the orbit ; in Melohmtha it is not per- 

 ceptibly distinguished from the hemisphere of the brain, nor is its dis- 



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