318 ELEMENTARY PHYSIOLOGY. [XXX. 



15. Carefully cut tbrough the anterior portion of 

 the fornix and turn it back, being careful not to 

 drag with it the choroid plexus; the choroid 

 plexus of each side will be seen to curve back 

 slightljs and join with the other in the median 

 line; the space left on either side between 

 the choroid plexus and the anterior pillars of 

 the fornix is part of the foramen of Monro. 

 The remaining part of the foramen of Monro 

 is the space in the median line between the 

 anterior portions of the choroid plexuses where 

 they join; this leads into the third ventricle 

 (cp. § 19); the foramen of Monro is thus roughly 

 a Y-shaped space, one limb communicating with 

 the third ventricle and each of the other two 

 with a lateral ventricle at the junction of the 

 body with the anterior cornu. 



16. The vascular membrane underneath the fornix 

 is the velum interpositum, this is seen to be 

 continuous anteriorly with the recurved ends of 

 the choroid plexuses about the foramen of Monro 

 and laterally with the whole length of the 

 choroid plexus of the lateral ventricles, in fact 

 the choroid plexuses are only the free borders of 

 the velum interpositum curving over the edge of 

 the fornix and over its posterior pillars. (It is to 

 be remembered that since there is an epithelial 

 membrane running from the fornix over the 

 choroid plexuses to the edge of the nucleus 

 caudatus, the lateral ventricle has no opening 

 except at the foramen of Monro.) 



