499 



{c.p.) and above and below it lie large veins {y). The lower surface 

 of the posterior commissure is covered, in the middle line, by the 

 peculiar high columnar epithelium of the sub-commissural organ (s.c.o.), 

 which is continued round the anterior end of the posterior commis- 

 sure for a short distance into the floor of the infra-pineal recess, 

 where it gradually loses its columnar character. Beneath the epithe- 

 lium are seen portions of Reissner's fibre {r.f.) whose branches, no 

 doubt, are connected with the 

 epithelial cells as in lower 

 types, and as in the cat to be 

 described later on. 



It will be noticed that 

 the epithelium of the sub- 

 commissural organ is thrown 

 into a transverse fold near its 

 anterior end, just before it turns 

 round over the anterior end of 

 the posterior commissure. To- 

 wards its hinder extremity it 

 appears to become discon- 

 tinuous, and two little separate 

 islands are formed, which have 

 become partially invaginated 

 into the underlying tissues 

 of the brain in the form of 

 small pits {pit). 



Fig. 2. Mouse. Transverse section through the region of the posterior com- 

 missure, taken at about the level of the line ab in text-fig. 1. X ^'^• 



c.p. choroid plexus; i:p.r. infra-pineal recess; p.c. posterior commissure; s.c.o. 

 sub-commissural organ ; s.p.r. supra-pineal recess ; v. veins. 



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 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 horse shoe-shaped groove, 

 lined by greatly elongated columnar cells and very sharply marked 

 ofi" from the ordinary ependyma of the iter below it, the latter ex- 

 hibiting a characteristic fold on either side where it joins the former. 

 The cells of the sub-commissural organ are probably provided with 



32* 



