ANATOMY OF A CYCLOSTOME BRAIN 653 



In the floor of the mid-brain the very complex ansulate com- 

 missure system occupies the thick massive floor plate as far for- 

 ward as the tuberculum posterius. 



The entire massive wall of the recessus mammillaris contains 

 commissural fibers. Anteriorly this commissural mass is some- 

 what thickened by fibers which appear to connect with the mas- 

 sive lateral walls of the infundibulum. This collection of fibers 

 is termed the post-infundibular commissure (figs. 3, 9, 10, 11, 

 com.pi.). 



Between the infundibulum and the postoptic recess is a fold of 

 the ventral wall of the brain which contains another mass of 

 commissural fibers, the preinfundibular commissure (figs. 3, 5, 

 com.pri.). 



The optic chiasrna and the postoptic commissures. The large 

 commissural mass which is commonly termed the optic chiasma 

 contains the decussation of the optic tracts and a much larger 

 collection of commissural and decussating fibers which in the ag- 

 gregate are termed the postoptic commissure, together with 

 many nerve cells. The latter elements form the plates of cells 

 in the central gray already referred to as crossing the medial 

 plane on the dorsal and ventral borders of the chiasma ridge 

 (fig. -5). 



The fibrous masses lie between the two cell plates and of these 

 masses the optic fibers form a relatively small proportion. The 

 optic fibers are coarser than the others and so can readily be 

 followed. Figure 5 illustrates their relations at their decussa- 

 tion, where they lie dorsally of the finer fibered postoptic 

 commissure, and figure 3 indicates by a heavy line the full extent 

 of the optic fibers in the medial plane. A cross section through 

 the middle of the chiasma ridge is shown in figure 6, where it 

 can be seen that the cell plates are considerably thicker than 

 farther forward. All of the fibers here present in the medial 

 plane belong to the postoptic commissure system, the optic tract 

 lying farther dorsally at the lateral surface of the brain. This 

 condition prevails throughout the remainder of the chiasma ridge 

 until its extreme posterior end is approached, where the dorsal 



