THE CEREBELLUM. 27> 



nerve the former are the thickest of all the nerves it gives off. Their 

 direction as well as origin depends upon the situation of the head, for 

 upon its horizontal position they spring further below from the cere- 

 brum, but upon its vertical position we find them originate from its lower 

 surface. Their length also stands in direct proportion to the form of the 

 oesophagus ; they are long in broad and expansive ones, and shorter in 

 narrower ones. This is peculiar to haxistellate insects, and in them 

 therefore both the ganglia lie closely together. We observe this 

 approximation of the two very distinctly in the bees, in which the 

 connecting cord is nearly deficient, so that the cerebrum and cerebellum 

 are quite contiguous, and there only remains in the middle between 

 both a small aperture for the oesophagus. These connecting cords of 

 the two brains very rarely give off auxiliary branches. I have observed 

 the only instance of this kind in Gryllus migraforius, in which a 

 smaller auxiliary branch originates at a little beyond half its length 

 upon the inner side, which is united with its opponent beneath the 

 oesophagus, running closely to that organ itself. Immediately in front 

 of their point of connexion each again gives off a smaller branch, which 

 runs back to the main connecting nerve of the two ganglia ( PL XXXI. 

 f. 7. d, d. and d*, d*.}. 



185. 



THE CEREBELLUM. 



The cerebellum (PL XXXI. and XXXII. B, B,) is generally a 

 cordiform or longitudinal ganglion ; it lies at the base of the cavity of 

 the skull, between the two projecting ridges of the previously described 

 internal skeleton of the head, and is entirely covered by the tentorium. 

 At the anterior portion of its lateral margin two strong nervous cords 

 originate from it, which rise to the cerebrum, running contiguously to 

 the tentorium, and enclose the oesophagus between them, forming the 

 nervous loop described above as encircling it. At its posterior end, 

 however, it again runs in two equal and very approximate filaments, 

 which pass through the occipital aperture, beneath the transverse bone 

 which divides it when present, out of the head into the thorax ; they 

 lie consequently very low in the neck, closely above the membrane of 

 the neck and the fiexor muscles of the head. They are the origin of 

 the ganglionic nervous cord which runs along the pectoral and ventral 

 sides of the body. 



Between these two connecting nerves of the cerebellum with the 



