VELUM INTERPOSITUM. 205 



pieces with terminal fringes in the latter ventricles (/). But the slit 

 through which the membrane enters the ventricle is closed by the lining 

 structure of that cavity being continued on the intruded part. 



Fig, 64, 



SECOND VIEW OF THK DISSKCTION OF THE BRAIN, THE FORNIX BEING CUT THROUGH 

 IN FRONT AND RAISED. (From a cast in the University College, London.) 



a, Fornix. d. Corpus stnatum. 



6. Hippocampus major. e> Optic thalamus. 



c. TsDuia hippocampi, or hinder crus of /. Choroid plexus. 



the fornix. g> Velum interpositum. 



PARTS IN THE MIDDLE LINE OF CEREBRUM. The student is now to 

 return to the examination of the parts in the centre of the brain, viz., the 

 fold of pia mater and its vessels, with the third ventricle. At the same 

 time the optic thalamus is to be seen. 



The velum interpositum (fig. 04, g) is the central part of the fold of 

 pia mater entering the great transverse fissure. Triangular in shape, it 

 has the same extent as the body of the fornix, and reaches in front to the 

 foramen of Monro. The upper surface is in contact with the fornix, to 

 which it supplies vessels. And the lower surface, looking to the third 

 ventricle, covers the pineal body, and a part of each optic thalamus : un- 

 derneath it in the middle line are the two choroid plexuses of the third ven- 

 tricle. Along each side is another vascular roll of the membrane (choroid 

 plexus). 



The choroid plexus of the lateral ventricle (fig. 64, /) is the red, some- 

 what rounded, and fringed margin of the piece of pia mater in the inte- 

 rior of the lateral ventricle, which extends from the foramen of Monro to 



