503 



III. The Sub-commissural Organ in Man. 



A mesocoelic recess, evidently homologous with that of the chim- 

 panzee, but very much smaller, is readily recognisable in transverse 

 sections of the adult human brain, where it appears to constitute the 

 last vestige of the sub-commissural organ, while in the five months* 

 fcetus this organ is still fairly well developed. 



In the foetus the sub-commissural organ is, indeed, rather better 

 developed than in the chimpanzee. As in that animal it begins 

 anteriorly in the infra-pineal recess, where the characteristic high 

 columnar epithelium, with deeply-seated nuclei, is well marked. As 

 it is traced backwards around the curve of the posterior commissure, 

 its paired nature becomes evident. The two halves diverge, and come 

 to lie widely separated in the roof of the iter, forming a pair of 

 small shallow grooves, as shown in text-fig. 7. Laterally these grooves 

 are not clearly marked off from the 

 general ependyma, which, as is well ... Vvöd.:. . 



known, is much more strongly 

 developed in the foetus than in the 

 adult, but is distinguished from the 

 epithelium of the grooves by the 

 much more superficially-placed nuclei, 

 Mesially the edges of the grooves 

 become more sharply defined (text- 

 fig. 7), the ependyma having practi- 

 cally disappeared from beneath the 

 commissure between the grooves 

 (compare cat, text-fig. 5). 



Towards the hinder end of the 

 posterior commissure these two grooves 

 unite again and become invaginated 

 into the roof of the iter to form a 

 mesocoelic recess exactly comparable 

 to that already described in the chim- 

 panzee, but more widely open below 

 (text-fig. 8). 



This recess is lined throughout 

 by the characteristic ependyma of 



the sub-commissural organ, and tunnels forwards into the brain 

 tissue above the posterior commissure for a distance of about 0.35 mm. 

 Its full length is rather greater than this owing to the tube be- 





Fig. 7. Human fcetus of about 

 five months. Transverse section 

 through the region of the posterior 

 commissure. X 45. 



e.p.e. ependymal epithelium of 

 iter ; m.r. mesocoelic recess ; p.c. 

 posterior commissure; s.c.o. sub-com- 

 missural organ. 



