CEREBEAL COMMISSUEES IN THE VERTEBRATA. 



487 



extremity of tlie optic thalamus by the most direct route, which happens to be situated 

 in the posterior wall of the foramen of Monro. These fornix-fibres then proceed across 

 the roof of the forebrain and form the commissura aberrans. A study of the accom- 

 panying' scheme will sliow that the commissure is placed at tlie exact situation in which 

 the narrow epithelial roof of the slit-like third veiitriclc passes into continuity with the 

 broad roof of the " aula," which expands in the lateral direction on each side to form the 

 choroid plexus. The latter passes through the foramen ol" Monro into the lateral 

 ventricle. 



Fis. 21. 



Schematic representation of a horizontal plan of the brain to iudicMto the situation of 



the cominissnfa al'irrans. 



In the introductory remarks I called attention to the fact that the pai"aterniinal 

 bodv is not limited by the lamina terminalis ; and I showed, by the lielp of drawings 

 of coronal sections, that a process or caudal prolongation of the paraterminal body 

 proceeds backward abov^e the foramen of ]\Ionro, and fuses with the dorso-cephalic 

 anii'le of the optic thalamus. lu order to )nake these dithcult and very complicated 

 relations intelligible, 1 have constructed a crude diagram (fig. 22, p. 488) of tlie 

 mesial surface of the forebrain (in part only) from wliich all epithelial folds Jiave been 

 removed. 



This scheme will explain ho^- the paraterminal body extends backward alcove the 

 lamina terminalis and above the foramen of Monro, and how this caudal prolongation 

 fuses with the optic thalamus. At the savne time the paraterminal body alfords a 

 matrix or "commissure-bed" for tlie comiiilasiira aherraiis just as it does for the 

 commissnra dorsoUs and commissura centralis. It was to this caudal extension of tlie 

 paraterminal l)ody that Edinger gave the name •' Fornlxleiste.'" Other writers called it 

 the '• fornix-body " or even the " fornix." The latter term has been so used because 



SECOND SEKIKS.— ZOOLOGY, VOL. VIII. 71 



