564 CAROLINE B. THOMPSON 



part of the brain and directly above the mouth opening. The 

 connection of the frontal ganglion and the labrofrontal nerves 

 is not shown in the diagram nor is the recurrent nerve which 

 runs backward from the frontal ganglion. 



The delicate unpaired nerve from the dorsal surface of the 

 frontal ganghon to the protocerebral lobes, and the labral nerves 

 from the ventral surface of the frontal ganglion, are figured but 

 not labeled. Posterior to the tritocerebral commissure the two 

 slender ventral connectives, v.c, run first backward (which can- 

 not be shown in this diagram), then downward, and unite to 

 form the large subesophageal ganghon, a single mass, consisting 

 of the fused mandibular, maxillary, and labial gangUa, and from 

 which arise the mandibular, maxillary, and labial nerves. Pos- 

 terior to this ganghon the thoracic connectives, not shown in 

 the figure, pass upward and then backward into the thorax. 



THE FINER STRUCTURE OF THE BRAIN 



/. The brain sheath 



The entire brain is surrounded by a membranous sheath com- 

 posed of a single layer of cells resembling mesenchym cells, their 

 fibrous processes making a continuous double walled membrane 

 between which the cell bodies and the nuclei are situated. 



//. The protocerebral lobes 



The protocerebral lobes (fig. 2, p.l.) form the central part of 

 the supraesophageal ganghon; they are continuous on their dor- 

 sal surface with the mushroom bodies, on their lateral surfaces 

 with the optic lobes, and on their ventral surface with the an- 

 tennal lobes, and are also connected by a slender nerve with the 

 frontal ganglion, fr.gn., which lies anterior and somewhat ven- 

 tral to the protocerebrum. The ocellar nerves, from the lateral 

 ocelli, and the fontanel nerve, from the frontal gland, also enter 

 the protocerebral lobes, and will be considered in more detail 

 under those headings. Like other parts of the brain the proto- 

 cerebral lobes consist of a central fibrous core and an outer in- 

 vesting layer of nerve cells. Most of these nerve cells are small 



