206 DISSECTION OF THE BRAIN. 



the extremity of the descending cornu. Its lower end is larger than the 

 upper. On its surface the choroid plexus is villous ; and the villi, minutely 

 subdivided, are covered by flattened nucleated epithelium, with the fat 

 granules and pigment in the cells. 1 



Vessels of the velum. Small arteries have been already traced to the 

 velum and the choroid plexus from the cerebral and cerebellar arteries (p. 

 175) : they are three on each side, and supply the surrounding cerebral 

 substance. The veins of the choroid plexus receive branches from the 

 ventricle, and end in the following. 



Veins of Galen. Along the centre of the velum are placed two large 

 veins with this name ; they begin at the foramen of Monro, by the union 

 of branches from the corpus striatum and the choroid plexus. Lying side 

 by side in the membrane they are usually united into one at the posterior 

 part of the velum ; and by this they join the straight sinus. 



Dissection. When the velum interpositum has been raised and thrown 

 backwards, the third ventricle will be visible (fig. 68). In reflecting the 

 piece of pia mater the student must be careful behind of the pineal body 

 ($r), which would otherwise be detached, as it is surrounded by the mem- 

 brane. On the under surface of the velum are the choroid plexuses of the 

 third ventricle. 



The choroid plexuses of the third ventricle are two short and narrow 

 fringed bodies beneath the velum, which resemble the like parts in the 

 lateral ventricle. 



The third ventricle is in the interval between the optic thalami (fig. 

 68). Its situation is in the middle line of the cerebrum, below the level 

 of the other ventricles with which it communicates ; and it reaches to the 

 base of the brain. Its boundaries and communications are the follow- 

 ing : 



The roof is formed by the velum interpositum and the fornix. The 

 floor is very oblique from behind forwards, so that the depth of the cavity 

 is about an inch in front and half an inch behind : it corresponds with 

 the parts at the base of the brain, which lie between the crura cerebri and 

 the median fissure (fig. 59), viz.", locus perforatus, corpora albicantia, tuber 

 cinereum, commissure of the optic nerves, and lamina cinerea. On the 

 sides of the cavity are situate the optic thalami (b). In front of the space 

 are the descending pillars of the fornix, with the anterior commissure of 

 the cerebrum (c) in the interval between them. Behind are the posterior 

 commissure (e) and the pineal body (g). Crossing the centre of the ven- 

 tricle, from one optic thalamus to another, is a band of gray matter the 

 soft commissure (c?). 



This space communicates with the other ventricles of the brain in the 

 following way: In front it joins eacli lateral ventricle through the fora- 

 men of Monro ; and in the foetus it opens into the fifth ventricle. Behind 

 is a passage beneath the posterior commissure into the fourth ventricle, 

 which is named aqueduct of Sylvius. At the lower part, in front, there 

 is a depression opposite the inf'undibulum (iter ad infundibulum). 



The lining of the ventricle (ependyma) is continued into the neighbor- 

 ing cavities through the different apertures of communication, and closes 

 the iter ad infundibulum. 



Gray matter of the ventricle. A stratum of gray matter covers most 

 of the surface of the ventricle. At the lower part of each optic thalamus 



1 Particles of brain sand, like that in the pineal body, are sometimes present 

 in the choroid plexus. 



