CORRELATING CENTERS IN THE DIENCEPHALON. 



273 



out into the retina. The third neuromere gives rise to the greater 

 part of the thalamus, the nucleus habenulae and an epiphysis. 

 The second neuromere forms the anterior part of the thalamus 

 and the hypothalamus. The recognition of the two main divisions 

 of the diencephalon and their genetic history leads the way to an 

 understanding of the morphological relations of its special nuclei. 



Dorsal decussation 



Tr. opticus 



Tr. strio-thalam. 



Nuc. magnocel- 



lularis 

 Comm. posterior. 



Tr. tecto-cerebel. 



Postoptic decuss. 

 Tr. lobo-bulbaris 



Tr. thal.-saccus Tr. lobo-epistriaticus 



Fig. 135. Transverse section through the posterior commissure of the sturgeon. 



The gross relations of the hypothalamus being established 

 it remains to discuss its functional relations as indicated by the 

 fiber tracts connected with it. The tracts which bring impulses 

 to the hypothalamus are primarily the gustatory and olfactory 

 tracts of the third order. Although the gustatory tracts have only 

 recently been demonstrated they are probably not less fundamental 

 18 



