THE NERVOUS SYSTEM 1409 



canipi ; {3) the ertlinentia collateralis, or pes accessorius ; and 

 (4) the choroid plexus of the descending comu. The hippocampus 

 major, or cornu Ammonis, is a prominent cur\'ed elevation which 

 traverses the entire length of the descending comu, accurately 

 adapting itself to its curves. It enlarges as it descends, and 

 beneath the amygdaloid tubercle it terminates in a swelling, which 

 is notched on the surface. This swelling is called the pes hippo- 

 campi. 



The hippocampus major is produced by the dentate or hippo- 

 campal fissure on the mesial surface of the cerebral hemisphere. 



The fimbria, or taenia hippocampi, is the continuation of the 

 posterior pillar of the fornix. It lies adong the iimer concave border 

 of the hippocampus major, to which it is attached, and it is com- 

 posed of white fibres, some of which form the layer on the surface 

 of the hippocampus major, called the alveiis. 



The emlnentia collateralis, or pes accessorius, is an elevation which 

 is situated in the angle between the descending and posterior 

 comua, where there is a small triangular space, called the trigonum 

 ventriculi. It extends backwards into the posterior comu, and 

 for a variable distance into the descending comu. 



The eminentia collateralis is produced by the central portion of 

 the collateral fissure on the tentorial surface of the cerebral hemi- 

 sphere. 



The choroid plexus of the descending cornu rests upon the surface 

 of the hippocampus major, and is continuous with that of the body 

 of the lateral ventricle. It is covered by the epend^Tna of the inner 

 wall of the descending comu, which it invaginates. \Vhen the 

 choroid plexus is removed its ependjinal covering comes away 

 along with it, and the choroidal fissure then becomes apparent. 



The choroidal fissure is situated between the fimbria and the roof 

 of the descending comu, and, curving round the back part of the 

 optic thalamus, it is traceable as far forwards as the foramen of 

 Monro of the same side. In the other direction it extends to the 

 lower extremity of the descending comu. 



It is produced by an infolding or invagination of the epithehal 

 mesial wall of the cerebral vesicle of one side over the choroid plexus 

 of the descending comu of the lateral ventricle. On either side it 

 is continuous with the lateral and lower part of the transverse fissure. 



When the choroid plexus is withdrawn from the descending comu 

 of the lateral ventricle, the epithehal or ependjTnal covering of the 

 plexus comes away with it, or is broken down. Under these circum- 

 stances the descending comu opens freely upon the exterior. 



The choroidal fissure is a complete fissure, and the internal prom- 

 inence corresponding to it is the choroid plexus in the descending cornu 

 of the lateral ventricle. 



Development. — ^The lateral ventricles represent the cavities of the primitive 

 cerebral vesicles. The choroidal fissure is developed as an invagination of 

 the mesial wall of the cerebral vesicle; and the choroid plexus is developed 

 from a growth of mesoblast into the choroidal fissure. 



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