Johnston, The Brain of Petromyson. 29 



outer fine-fibered portion runs along the ventro-lateral surface 

 of the medulla and seems to extend into the cord. These are 

 therefore the tractus bulbo-tcctalis and tractus tecto-bulbaris 

 respectively. Each has a portion crossing through the ansulate 

 commissure, as in Acipenser. The tract passing forward is the 

 tractus tecto-lobaris. The arrangement of all these tracts is 

 simpler than in Acipenser and the tractus tecto-lobaris is much 

 smaller and probably has no crossed portion. 



From the cephalic portion of the tectum a tew fibers enter 

 the posterior commissure to cross to the opposite side. These 

 are probably to be regarded as an isolated portion of the dorsal 

 decussation, since they seem to enter the tectum of the other 

 side. 



The cells which accompany the optic tracts at the level of 

 the posterior commissure and a little farther cephalad are far 

 removed from the cavity and are all stellate and smaller than 

 the horizontal or vertical cells of the tectum proper. I could 

 trace the neurites. of only two cells. One of these crossed to 

 the opposite side in the posterior commissure, the other ran 

 ventrad with the fibers of the commissure of the same side. 



The nucleus which gives origin to Reissner's fiber as de- 

 scribed by Sargent ('01) was not seen, although the fiber itself 

 is very distinctly seen in longitudinal sections (Fig. 2). 



b. The central grey and posterior commissure (Figs. 



15. 30). 



The nucleus of the III nerve and the situation of three large 

 cells which give rise to Mullerian fibers have been described 

 above. The cells of the central grey probably belong to two 

 distinct categories. Some are doubtless to be compared with 

 the commissural and tract cells of the cord and medulla, 

 others send their neurites to the posterior commissure. There 

 is a considerable collection of cells adjoining the cavity just 

 lateral and ventral to the commissure, and both these and a 

 large number of more distant cells in the central grey of the 

 thalamus and mid brain send their neurites into the commissure. 

 Some cells scattered in the fiber zone also belong to this cate- 

 gory. - The neurites of these cells are always fine at their ori- 



