ANATOMY OF THE NERVE CENTRES. 445 



thalamencephalon, mesencephalon, and part of the meten- 

 cephalon of Vertebrates. 



The Thalamon consists of a pair of thalamic lobes. These give 

 origin in the Crayfish to the greater part of the optic peduncle 

 (o), and are intimately connected below with the ganglia 

 of the antennules, which, like the antennal ganglia of insects, 

 have a special and peculiar structure, identical with that of the 

 cortical layers of the olfactory lobes of Vertebrates ; a fact 

 placed beyond dispute by the researches of Dietl, Bellonci, 

 and Viallanes. This structure is so peculiar that I regard it as 

 indubitable that they are olfactory in function ; a view which 

 I shall hereafter show is confirmed by the structure of the 

 antennas in many insects, and by their comparative develop- 

 ment in various families. 



In front of and between the olfactory and below the thalamic 

 lobes, in the Crayfish, there is a minute tongue-like unpaired 

 process of reticular substance covered thickly by ganglion cells 

 (Fig. 55, cl). This process is hollow and communicates with 

 a ventricle. I regard it as the representative of a pair of 

 convoluted lobes in the Blow-fly (PI. XXIX., Fig. i, cl), and 

 propose to term it the procerebron. This structure has not, 

 so far as I know, been previously seen in the Crayfish. Thus 

 the thalamon may be said to consist of (i) a pair of thalamic 

 lobes (ill), (2) of the olfactory ganglia (a), and (3) of the pro- 

 cerebral lobe (c/). 



The Mesocerebron (in, in) consists of a pair of spherical ganglia 

 united by a thick transverse commissure (Fig. 54, /). The 

 medulla of the mesocerebron is divided into two masses in 

 each lateral ganglion by a lamina of radiating fibres, which 

 terminate in a thick layer of the cellular cortex, the cells of 

 which are small and like those of the corpus fungiforme of an 

 insect's brain. 



It appears to me that the lamina of radiating fibres, which 

 is gathered up within the brain into a well-marked bundle 

 (Fig. 54, 2), represents the peduncle and trabecula of the 

 corpus fungiforme. Like the trabecula, this bundle gives off 

 definite fasciculi of fibres to all the other parts of the brain. 



