MIDBEAIN AND THALAMUS OF NECTURUS 255 



with all parts of the tectum mesencephali are directed ventral- 

 ward and forward into the diencephalon to reach the postoptic 

 commissure in the chiasma ridge. These will next be consid- 

 ered. (For a summary and discussion of the components of 

 the postoptic commissure, see page 276.) 



12. The tractus tecto-thalamicus et hypothalamicus cruciatus 



This is the largest group of fibers in the postoptic commissure 

 complex. Its fibers are related to the tectum mesencephali 

 for its entire length, and apparently they comprise several 

 components of different functional significance, depending upon 

 the part of the tectum with which they are related. These 

 fibers in Necturus I divide into two chief groups which may be 

 termed the anterior and posterior parts of the system, the pars 

 anterior being related to the optic part of the tectum (col- 

 liculus superior) and the pars posterior to the non-optic part of 

 the tectum (colliculus inferior). 



1) Pars anterior. There are a very few coarse myelinated 

 fibers related to the most dorsal part of the neuropil which re- 

 ceives terminals of the optic tract. They accompany the mar- 

 ginal optic tract almost to the optic chiasma, where they separate 

 and decussate somewhat farther dorsally and caudally than 

 the optic fibers. They form the most rostral member of the 

 postoptic commissure complex in the chiasma ridge (figs. 2 to 

 11, tr.i.th.h.c.a.). Their farther course is not altogether clear. 

 They may include true commissural fibers between the optic 

 tecta, but most of the fibers (which are few in number but very 

 coarse and heavily myelinated and therefore distinguishable 

 from all others in the postoptic commissure) appear to connect 

 with the lateral parts of the hypothalamus and adjacent motor 

 tegmentum. The coarsest of these fibers, which decussate 

 farther caudad than the others, can be followed after their de- 

 cussation along the extreme lateral surface of the hypothalamus 

 as far back as the tuberculum posterius. 



A very few similar heavily myelinated fibers are described 

 by Hirsch-Tabor ('08, p. 724 and fig. 1, C) in Proteus, where 



