THE NERVOUS SYSTEM. 299 



Its upper surface is partly covered by the fornix. Its 

 under surface forms the roof of the descending horn of the lateral 

 ventricle. 



Its posterior and inferior part exhibits two rounded emi- 

 nences, the external and internal geniculate bodies. Its anterior 

 extremity forms the posterior boundary of the foramen of Monro, 

 which foramen connects the two lateral ventricles with the third. 



Velum inter positum, a vascular membrane, reflected from 

 the pia mater into the interior of the brain through the trans- 

 verse fissure. It passes beneath the posterior border of the corpus 

 callosum and fornix, and above the corpora quadrigemina, the 

 pineal gland, and the optic thalami. It forms the roof of the 

 third ventricle. Its anterior extremity passes on each side into 

 the corresponding lateral ventricle, forming the anterior extrem- 

 ity of the choroid plexus. The vascular fringes of the velum 

 interpositum projecting into the third ventricle are called the 

 choroid plexuses of the third ventricle. 



It has two veins, the venae Galeni, which run along its under 

 surface and are formed by the veins of the choroid plexuses and 

 the venae corporis striata. The venae Galeni unite to form a 

 single trunk and empty into the straight sinus. 



The posterior cornu of the lateral ventricle runs into the 

 substance of the posterior lobe. On the floor of this horn is an 

 eminence corresponding to a sulcus between two convolutions, 

 and called the hippocampus minor. 



Between the posterior and middle horns is another emi- 

 nence the eminentia collateralis, or pes accessorius. 



The hippocampus major, or cornu ammonis, a white emi- 

 nence running the entire length of the floor of the middle horn. 

 This eminence is the doubled-in surface of the gyrus fornicatus. 



The lower extremity of the hippocampus major is called 

 the pes hippocampus. 



The fascia dentata, the gray and serrated edge of the middle 

 lobe. It is really external to the cavity of the middle cornu. 



THE THIRD VENTRICLE is a mere fissure in the median line 

 of the cerebrum, situated between the optic thalami. It com- 

 municates with the lateral ventricle by the foramen of Monro 

 and with the fourth ventricle by the iter a tcrtio ad quart u in 

 ventriculum. The cavity is crossed by three commissures the 

 anterior commissure, a white, rounded cord; the middle or soft 

 commissure, composed of gray matter; and the posterior com- 

 missure, a white band connecting the two optic thalami. It is 

 bounded by the following structures ; - 



