CORRELATING CENTERS IN THE DIENCEPHALON. 



277 



tuber cinereum have been followed only into the central gray 

 of the thalamus (Fig. 137). This is the direction taken by the 

 tr. lobo-bulbaris in lower vertebrates. Another tract from the 

 hypothalamus which is not known in mammals and but poorly 

 understood in lower vertebrates, is an ascending tract to the 

 forebrain, the tractus lobo-epislriaticus. 



A second olfactory conduction path, already described for 

 fishes, is found in mammals; namely, that by way of the nucleus 

 habenulae. The tract from the forebrain to the nucleus habenulae 

 comes partly from the secondary olfactory nucleus, the nucleus 

 thaeniae or nucleus amygdalae, and in part from the hippocampus. 

 Those from the hippocampus are a part of the fornix system. 



Tr. olfacto-habenularis 



Fig. 139. A scheme to show the embryological relations of the nucleus habenulae 

 and the inferior lobes in fishes, i, 2, neuromeres. 



The tract forms' a part of the striae medullaris and enters the 

 nucleus habenulae, where it contributes to the habenular commis- 

 sure as in lower forms. From the nucleus habenulae the tractus 

 habenulo-peduncularis descends to the base of the mesencephalon, 

 where the paired tracts form an intricate decussation and end in 

 the corpus inter pedunclare. 



The fact that the two olfactory conduction paths diverge so 

 widely seems at first sight difficult to understand. The accom- 

 panying diagram will serve to show how this has come about. In 

 Fig. 101 the olfactory paths are sketched into the outline of a 

 sagittal section of a fish brain, and in Fig. 139 a reconstruction is 

 given in which the several centers are drawn in their embryonic 



