428 C. JUDSON HERRICK 



5. The tractus olfacto-habenularis system. Primitively, as in 

 cyclostomes, this tract runs in diffuse formation from practically 

 all parts of the secondary olfactory center to converge in the stria 

 medullaris and reach the habenula, part of its fibers first decus- 

 sating in the superior commissure. In the Amphibia, with the 

 further differentiation of the caudal part of the telencephalon, 

 the relations become very complex, though the same in prin- 

 ciple. There are five components (paragraphs 6 to 10 below) 

 of this system, of which the largest comes from the nucleus pre- 

 opticus and constitutes the tractus olfacto-habenularis in the 

 restricted sense. 



6. The tractus olfacto-habenularis lateralis arises chiefly from 

 the anterior part of the preoptic nucleus (see p. 432) and passes 

 upward into the stria medullaris laterally of the lateral forebrain 

 tract (fig. 19). 



7. The tractus olfacto-habenularis medialis arises chiefly from 

 the pars magno-cellularis of the preoptic nucleus and ascends 

 internal to the lateral forebrain tract (fig. 18). 



8. The tractus septo-habenularis arises in the nucleus medianus 

 septi (it is much larger in Necturus) and passes backward close 

 to the ventricular ependyma to cross the dorsal surface of the 

 anterior commissure and enter the stria medullaris in company 

 with the tractus cortico-habenularis medialis. 



9. The tractus cortico-habenularis lateralis is one of the largest 

 components of the stria medullaris. It passes from the posterior 

 pole in company with the commissura pallii posterior (fig. 18). 



10. The tractus cortico-habenularis medialis consists of a few 

 fibers which pass from the caudal end of the primordium hippo- 

 campi directly into the stria medullaris. Some of its fibers prob- 

 ably cross in the commissura pallii anterior. 



All of the components of the tractus olfacto-habenularis just 

 enumerated (numbers 6 to 10) occur in the frog and all but the 

 last in the reptiles. 



11. Tractus cortico-thalamicus. This is a sparse collection of 

 medullated fibers accompanying the tractus cortico-habenularis 

 medialis to the stria medullaris; but instead of turning dorsally 

 into the habenula, they continue backward into the thalamus 



