INTERIOR OF THE CEREBRUM. 



667 



The corpus fimbriatum or fimbria (tcenia hippocampi) has already been men- 

 tioned as a part of the posterior pillar of the fornix. It consists of a narrow white 

 band, which is placed immediately below the choroid plexus, and is attached by its 

 deep surface to the white matter (alveus) of the hippocampus major as it courses 

 through the descending cornu of the lateral ventricle. It can be traced as far as 

 the uncus or hook of the hippocampal gyrus. Its inner margin is free, and rests 

 upon the dentate convolution, from which it is separated by a slit-like fissure, the 

 fimbrio-dentate fissure. Its outer margin is attenuated and irregular, and forms 

 the line along which the ependyma is reflected over the choroid plexus as the 

 latter is invaginated through the inferior part of the transverse fissure. When the 

 choroid plexus is pulled away it carries the ependyma with it, and the descending 

 horn opens on to the surface of the brain through the transverse fissure. If now 

 the inner border of the corpus fimbriatum be raised, a notched band of gray 

 matter, the gyrus dentatus, will be exposed ; this has already been described as 

 forming part of the limbic lobe (page 653). 



Fornix 



Optic thalamus 

 Corpora quadrigemina 



FIG. 357. Diagram showing the mode of formation of the velum interpositum. 



The choroid plexus is a highly vascular, fringe-like structure, which is situated 

 partly in the body and partly in the descending cornu of the lateral ventricle. It 

 will be desirable to consider these two portions separately, in order to get a just 

 conception of how they are formed. 



The portion in the body of the ventricle is the vascular, fringed border of a 

 triangular fold of pia mater, the velum interpositum, which lies on the under 

 surface of the fornix and forms the roof of the third ventricle. It will be remem- 

 bered that the developing brain vesicles are covered by pia mater. As the pro- 

 longation from the first vesicle, which is to form the cerebral hemispheres, increases 

 in size, it grows backward and downward and covers the other vesicles, with the 

 result that the pia mater covering the hemisphere comes in contact with that cover- 

 ing the upper surface of the second vesicle (Fig. 357). 1 A portion of the two 

 layers which are in contact forms the velum interpositum. Immediately above is the 

 body of the fornix, which is formed by the fusion of the cerebral hemispheres in 

 the middle line and below is the cavity of the second vesicle (the third ventricle), 

 with the optic thalamus on either side (Fig. 359). Just beyond the free lateral 

 border of the fornix, between it and the trenia semicircularis, is a portion of the 



1 Tn the diagram the two layers are represented as being separated from each other, for the sake 

 of clearness. 



