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



525 



The elongated, radially arranged cells lining the recess are about 0 02 mm. 

 in length. 



Behind the recess the ependyma of the sub-commissural organ passes 

 gradually into the general ependyma of the iter. 



In sections througli the region of the posterior commissure in the brain of 

 the adult human subject the sub-commissural organ is seen to have almost 

 entirely vanished. The ependyma covering the ventral surface of the 



et> 0 



Fig. 9. — Human foetus of about five mouths. Transverse Section through the Mesocoelic 

 Recess, slightly anterior to the section represented in fig. 8. x 320. 



m. r., niesoccelic recess ; s.c. o., sub-commissural organ, forming the epithelial 

 lining of the mesocoelic recess. 



commissure consists apparently of somewhat flattened cells with conspicuous 

 nuclei, and differs very little from the general lining epithelium of the rest 

 of the iter ; the only difference being that upon the side walls of the iter the 

 nuclei are more crowded, and possibly the epithelium slightly more cubic or 

 columnar, but much less distinctly so than in the foetus. The ependyma 

 beneath the posterior commissure is continuous round the anterior surface 

 of the latter with that which lines the infra-pineal recess, and in no wise 

 differs from it. There is thus upon the surface of the commissure nowhere 

 any indication of that characteristic elongatedly columnar epithelium that 

 marks the sub-commissural organ in lower types. 



At the hinder limit of the posterior commissure, however, about the middle 

 line, there is an irregular thickening of tlie ependymal epithelium, and above 

 this an irregular cavity (Plate 14, fig. 2, m.r.) occurs in the tissue of the brain, 

 lined unevenly by the remains of an epithelium, with numerous scattered 

 nuclei, exactly like that on the roof of the iter beneatli it. No opening could 

 be found putting this cavity into communicatioa with the cavity of the iter, 

 but numerous scattered nuclei connect the two epithelia, and suggest the 

 remains of such a communication. 



