CEREBRAL COMMISSURES IX THE YERTEBRATA. 



465 



In this figure, however, there are certain features which have not hitherto been 

 represented. The most imjiortant of these is the little epithelial pocket which constitutes 

 the recessus superior. It is placed upon the upper surface of the dorsal commissure 

 between the mesial extremities of the fasciie deutatae. It is unnecessary for me to 

 explain how the fascia dentata reaches this position, which to the human anatomist is so 

 bizarre; because in several earlier communications I have explained this peculiar 

 phenomenon (more particidarly in Journal of Anatomy and Physiology, vol. xxxii. — 

 especially figure 3, page 30, which represents the analogous arrangement in 

 Fhascolarctos). 



■ ' Fis. 12. 



/■<^- 



cat. 



CM pans.. L.p. 



Coronal section of the cerebral hemispheres of Omitliorliyadn's. x about 21. The pointer from the letters c.d. 

 actually ends in the alveus hippocampi, from which the cominissiira dorsalis is derived. 



If the recessus superior be examined with a higher degree of magnification, its exact 

 relations will be more readily appreciated. But certain features are more clearly 

 demonstrated in a section Avhich passes immediately in front of the dorsal commissui-e 

 and, while missing the crossing fibres, passes through the grey matrix of this commissure. 

 The next figure represents the superior recess from such a section (fig. 13). 



In tliis section a solid mass of grey matter may be seen resting upon the dorsal aspect 

 of the ventral commissitre in the mesial plane, and extending upward on each side to 

 fuse wdth the ventral margin of the hippocampal formation and its specialized fringe of 

 fascia dentata. This mass of grey substance represents the posterior extremities of the 

 corpora paraterminalia Avhicli have fused the one with the other in front of the lamina 

 terminalis. The thickening of the lamina terminalis, which is thus produced at the 

 expense of the paraterminal bodies, corresponds to the bridge of grey substance whicli 

 forms the matrix for the two cerebral commissures. 



In an earlier contribution I called this the " commissure-bed." Into this " commissure- 

 bed " the fibres coming from the hippocampus — in other w^ords the fornix — pass, and 



68* 



