ON THE BRAIN OF THE COCKROACH. 345 



the relations of these orders, based chiefly upon the degree of 

 development of the mushroom bodies. 



Blatta {Periplaneta) orientalis. 



General form of the brain. — When the chitinous covering 

 of the upper and front part of a cockroach's head is removed, 

 together with the tissues which lie just within it, the brain, 

 or supra-oesophageal ganglion, is displayed as a pearly white 

 body, occupying but a small portion of the cavity of the 

 head (fig. 1). In this view the brain is seen to 

 consist of two rounded masses above, separated from each 

 other by a deep median fissure. From the outer sides of 

 these hemispheres, as they might be termed, the large nerves 

 are given off to the eyes {op-). Below are two smaller 

 rounded masses, marked ofif from the upper ones by a depres- 

 sion, these are the antennary lobes (anl,), from the outer side 

 of each of these a nerve passes off to one of the antennse. A 

 side view of the head dissected so as to expose the brain 

 (fig. 2) shows the latter to be placed very near the front 

 wall, while the space behind it is occupied to a large extent 

 by the muscles of the jaws. 



At first sight the only nerves given off from this upper 

 division of the brain seem to be the optic and antennary 

 nerves, but I have now been able to trace four other pairs ; 

 these, however, are very small. 



(1.) On more than one occasion, when opening the head 

 of a cockroach, I have observed a very delicate white fibre 

 passing from the front surface of the brain towards the 

 front wall of the head ; but thinking it was merely a tra- 

 cheal vessel, I had not troubled to trace its distribution. 

 After seeing Dr. Flogel's statement that a nerve passes out 

 from the front of the brain on each side, in the region where 

 I had noticed this white fibre, I searched again, and now 

 had the satisfaction not only of finding the nerves in the 

 position indicated (fig. 1 mvs), but also of tracing them most 

 clearly to those peculiar oval, silvery patches, which are 

 situated on the front of the head, just above and within the 

 antennee {ws.). It appeared to me that one of these nerves, 

 before reaching the silvery patch, gave off a branch which 

 passed round to the side of the brain, just above the optic 

 nerve ; but I could not trace it upon the opposite side, and 

 I failed altogether to see it in another specimen. 



(2.) Another nerve is to be found passing off from just 

 underneath the antennary lobe on each side (figs. 1 and 2 

 amn.), and these I have been able to trace to the muscles of 



