MIDBRAIN AND THALAMUS OF NECTURUS 275 



where the following components were described: (1) decussation 

 of the roots of the IV nerve, (2) decussation of fibers of the 

 mesencephalic root of the V nerve, (3) probably fibers of the 

 tractus tecto-cerebellaris, (4) unmyelinated fibers of the com- 

 missura cerebelli lateralis, (5) fine myelinated fibers of the com- 

 missura cerebelli, (6) coarser myelinated fibers of the commissura 

 cerebelli derived from the tractus spino-cerebellaris. In fishes 

 the decussatio veli contains a large commissure between the 

 secondary visceral nuclei (Rindenknoten of Mayser). I have 

 not found this in Necturus, though the secondary visceral 

 nucleus is here very small and such a tract might easily be over- 

 looked. 



The ventral decussations in the floor of the midbrain form a 

 very complex system which is the direct forward continuation 

 of the ventral commissure of the medulla oblongata and isthmus 

 regions, with a slight interruption at the fovea isthmi in the 

 region of the III nerves. The portion of this system lying below 

 the fovea will here be termed the ventral tegmental commissure; 

 the portion lying in front of the fovea will be termed the com- 

 missure of the tuberculum posterius. 



Ventral tegmental commissure 



Whether this region contains any true commissural fibers in 

 Necturus is not clear. The components here enumerated are 

 all decussations. These include the following crossed tracts: 

 (1) brachium conjunctivum (p. 251), (2) tractus tecto-bulbaris 

 cruciatus (p. 252), (3) tractus tecto-peduncularis superficialis 

 (p. 253), (4) tractus tecto-peduncularis profundus (p. 254), (5) 

 tractus tegmento-interpeduncularis (p. 272). 



Commissura tuberculi posterioris 



In the region of the tuberculum posterius, that is between 

 the fovea isthmi and the infundibulum, there is a dense accumu- 

 lation of commissural and decussating fibers of very diverse 

 sorts, derived from the midbrain, the epithalamus, the thalamus, 

 and the hypothalamus. Some of the tracts which cross in 



