CORRELATING CENTERS IN THE DIENCEPHALON. 269 



CHAPTER XVn. 



CORRELATING CENTERS IN THE DIENCEPHALON 

 (Continued). 



4, THE OLFACTORY AND GUSTATORY APPARATUS. 



The centers considered in the last chapter have to do with 

 somatic sensory impulses and are all differentiated from the 

 dorsal part of the mesencephalon and diencephalon. The centers 

 now to be considered have to do with visceral sensory impulses 

 and lie, with one exception, in the ventral part of the diencephalon. 

 It was shown in Chapters IX and X that gustatory impulses are 

 carried to the inferior lobes of the diencephalon by a tract from 

 the superior gustatory nucleus and that olfactory impulses come 

 to the same region in two or three isolated bundles of the tractus 

 olfacto-hypothalamicus. 



It is important to understand the exact limits of the hypo- 

 thalamus in fishes and to determine the corresponding structures 

 in higher vertebrates. In all fishes a pair of rounded lobes, the 

 inferior lobes, containing a wide extension of the third ventricle 

 project ventro-caudally behind the optic chiasma. In cyclostomes 

 and selachians these lobes are relatively small and have a simple 

 and primitive structure, but in ganoids and bony fishes they are 

 greatly expanded and show a somewhat higher histological develop- 

 ment, Caudo-ventrally the wall of the lobe is produced into a 

 median thin-walled and vascular sac known as the saccus vascu- 

 losus. The caudal wall of the lobes above the opening of the 

 saccus shows special characters and is known as the corpus mam- 

 millare. The relations of these structures may be seen in sagittal 

 sections of the brain (Figs. 2, 11, and Chap. XVIII). 



The whole area which shows the structural characteristics 

 of the inferior lobes and is related to olfactory and gustatory tracts 

 may extend somewhat beyond the lobes proper, and the limits 



