130 THE NERVOUS SYSTEM [CH. 



their prolongations. It gives rise internally and posteriorly to 

 the optic nerve. 



The Optic Nerve. This is a short but very distinct nerve, 

 formed of two separate bundles of nerve-fibres. Of these, the 

 superior optic bundle arises from the posterior face of the external 

 capsule of the opticon. and enters the brain by its upper anterior 

 region. It completely penetrates the cortex of the procerebral 

 lobe, and enters the medulla of the protocerebrum, where it 

 terminates in the optic tubercle (fig. 56, ot). 



The inferior optic bundle is much larger, and arises from the 

 whole of the internal capsule of the opticon. It enters the brain 

 by its lower lateral region (fig. 56 E, op). 



The Suboesophageal Ganglion (figs. 55, 56, sog). 



This is a large and well-formed ganglion situated below the 

 anterior portion of the brain, and connected with the tritocerebrum 

 by the short stout circumoesophageal commissures (co). It is 

 very compact, but still shews, in sections, evidence of its paired 

 origin. As in the thoracic and abdominal ganglia, the nerve-cells 

 are chiefly collected ventrally below the nerve-fibres. This 

 ganglion gives off three pairs of nerves, (i) the mandibular nerves, 

 from its extreme upper anterior corners ; (ii) the maxillary nerves, 

 from its lower anterior corners, at a level slightly behind (i) ; 

 (iii) the labial nerves (fig. 56 c, Ibn), which arise latero-ventrally 

 from the broadest part of the ganglion, well behind (ii). 



This ganglion narrows posteriorly, and finally gives origin to 

 the two exceedingly slender ventral nerve-cords, running very 

 close together backwards through the lower half of the posterior 



foramen, into the neck and prothorax. 







The Ventral Nerve-Cord (fig. 54). 



The two fine nerve-cords forming the double ventral cord may 

 be either distinct enough to be seen separately under a low power, 

 or they may be so closely united (in the abdomen) as to appeal- 

 single, though of course their double nature is easily seen in 

 transverse sections. These conditions appear to be correlated, 

 not with the degree of specialization of other parts, so much as 



