370 NERVOUS SYSTEM. 



several sections by transverse fissures, which give it an undu* 

 lated appearance. 



A transverse incision of the Hippocampus Major shows that 

 it is a body of cineritious matter, covered on its surface by a 

 thin layer of medullary substance. 



The Third Ventricle, ( Ventriculus Tertius.) When the fornix 

 is separated from its anterior crura and turned over backwards, 

 the process of pia mater, called Velum Interpositum, is found 

 between it and the optic thalami. This process is of a triangu- 

 lar shape, resembling the fornix, and is about the same size; it 

 is insinuated into its place from the surface of the brain, under 

 the posterior margin of the corpus callosum. Its lateral margins, 

 which project beyond the corresponding ones of the fornix, are 

 formed by a congeries of convoluted vessels, constituting the 

 Plexus Choroides. 



The Plexus Choroides, may be traced from the Pes Hippo- 

 campi along the corpus fimbriatum to its position on the mar- 

 gin of the velum interpositum ; and insinuates itself from the 

 bottom of the cerebrum between the pons varolii and the con- 

 volution forming the Hippocampus Major; but when it reaches 

 the anterior end of the fornix its convoluted character ceases, 

 and it terminates, on each side, in a skigle vein, (Vena Galeni,) 

 which runs from before backwards, in a straight line, near the 

 middle of the velum interpositum. The vein, finally, unites 

 with its fellow to form a single trunk, which runs into the fourth 

 sinus of the dura mater. 



This Velum Interpositum, called also Tela Choroidea, adheres 

 . very strongly to the fornix by means of small vessels: it may be 

 raised with less difficulty from the thalami, though it serves to 

 keep the third ventricle closed above, with the exception of the 

 part just behind the crura of the fornix, where the third and the 

 lateral ventricle communicate by the foramen of Monro. The 

 pineal gland is entangled in its posterior part, being placed be- 

 low it, and is generally torn from its peduncles when the tela is 

 raised up. It is at this point that the tunica arachnoidea may 

 be traced into the cavity of the lateral ventricles, according to 

 Bichat.* 



* Some doubts, I have stated before, may be reasonably raised on this point of 

 anatomy, as the evidence is seldom or never satisfactory to the full extent, and as 



