Dissection of Sheep's Brain. 175 



the Fornix across anteriorly, just above the Foramen of 

 Monro, and reflect the posterior portion backwards. 



When the posterior part is raised the Fornix will be found 

 to be supported on a vascular membrane, named Velum Inter- 

 positum ; the apex of the Fornix will be seen to end in two 

 processes, the Anterior Crura, separated from each other by 

 a narrow interval, and which afterwards descend in a curved 

 crescent form to the Corpus Albicans, where they each make a 

 turn like half the figure 8, and furnish a white envelope 

 to the grey matter of that body. The Foramen of Monro will 

 be seen to be merely an enlarged portion of an interval that 

 exists between the whole under surface of the body of 

 the Fornix and subjacent structures ; and the Choroid Plex- 

 uses will be seen to have their anterior origin from the bifur- 

 cated anterior extremity of the velum interpositum (just 

 posterior to the anterior crura of the Fornix), and also to be 

 connected with its lateral margins. By removing the Choroid 

 Plexus the lateral ventricles will be seen to descend towards 

 the middle line, where they afterwards join together and lead 

 into the upper part of the third ventricle. The Yelum Inter- 

 positum is reflected from the Pia Mater into the Interior of the 

 Brain through the transverse fissure*, passing beneath the pos- 

 terior rounded border of the Corpus Callosum and Fornix, and 

 above the Corpora Quadrigemina, Pineal Gland, and Optic 

 Thalami : it separates the under surface of the body of the For- 

 nix from the cavity of the third ventricle, and its posterior 

 border invests the Pineal Gland. The arteries of the Yelum 

 Interpositum are derived as well from the Choroid artery as 

 from the Superior Cerebellarf Artery, which enters from be- 

 hind, beneath the Corpus Callosum. Its veins, the VencB Oaleni, 



* Vide infra, p. 176. 



t The Superior Cerebellar Arteries arise near the termination of the Basilar 

 artery : they wind round the Crus Cerebri, close to the fourth nerve, and in ad- 

 dition to supplying the Pia Mater covering the Cerebellum, also give several 

 branches to the Pineal Gland and Velum Interpositum. 



