1910.] On a Mesocoelic Recess in the Human Brain, etc. 519 



are formed, which have become partially invaginated into the underlying 

 tissues of the brain in the form of small pits {int). 



The transverse sections represented in text-figs. 2 and 3 will serve to 

 further elucidate the relations above described. The lines ah and cd in 

 text-fig. 1 indicate approximately the levels at which these two sections are 



Fig. 3. — Mouse. Trans%-erae Section through the region of the Superior (Habenular) 

 Commissure at about the level of the line cd in fig. 1. x 60. 



c. c, corpus callosum ; h. g., habenular ganglion ; i.p. r., infi-a-pineal recess ; 

 s. c, superior commissure ; s.c. o., sub-commissural organ ; s.p. r., supra-pineal recess ; 

 v., veins. 



taken (all the sections are, of course, drawn from actual preparations). 

 In these sections we may confine our attention to the sub-commissural organ 

 (s.c.o.). Text-fig. 2 shows this organ in the form of a very conspicuous 

 horseshoe-shaped groove, lined by greatly elongated columnar cells and very 

 sharply marked off from the ordinary ependyma of the iter below it, the latter 



