7P,4 



DISSECTION OF THE BRAIN. 



Vessels of 

 the velum 



arteries ; 

 veins ; 



with veins 

 of Galen. 



Dissection 



Choroid 

 I^lexnses of 

 third 

 ventricle. 



Third 

 ventricle 



is near base 

 of brain. 



Roof. 



Floor. 



Parts on the 

 sides, 



in front 

 and behind. 



at the anterior extremity of the velum interpositum, where they are 

 similarly excluded from the foramina of Monro by the epithelial 

 lining.* 



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

 the velum and the choroid plexus from the cerebral and cerebellar 

 arteries (pp. 720, 721 and 724). There are two main ones on 

 each side, anterior and posterior clioroid, and they supply the sur- 

 rounding 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 through 

 this they pass out beneath the splenium of the corpus callosum and 

 enter the straight sinus. 



Dissection. When the velum interpositum has been raised and 

 thrown backwards, the third ventricle will be opened (fig. 277). 

 In reflecting the velum the student must be careful not to detach 

 the pineal body behind, which is surrounded by the membrane and 

 rests on the fore part of the anterior quadrigeminal bodies (fig. 277, g). 

 On the under surface of the velum are seen the choroid plexuses 

 of the third ventricle. 



The CHOROID PLEXUSES OF THE THIRD VENTRICLE are two short 

 and narrow fringed bodies below the velum, which resemble the 

 like structures in the lateral ventricle. 



The THIRD VENTRICLE is the narrow interval between the optic 

 thalami (fig. 277). Its situation is in the median plane of the 

 cerebrum, below the level of the lateral ventricles, with which it 

 communicates ; and it reaches to the base of the brain. Its 

 boundaries and communications are the following : 



The roof is formed by the velum interpositum with the choroid 

 plexuses, above which is the fornix. The floor (fig. 274) is very 

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

 about three-quarters of an inch in front and half as much behind : 

 its hinder part is formed by the united tegmenta of the crura cerebri ; 

 and in front of these it corresponds with the parts at the base of 

 the brain, which lie between the crura cerebri, viz., the posterior 

 perforated space, the corpora albicantia, the tuber cinereum with 

 the infundibulum, and the optic commissure. On the sides of the 

 cavity are situate the optic thalami and the anterior pillars of the 

 fornix (fig. 274). In front of the space are the anterior com- 

 missure and the lamina cinerea. Behind are the posterior com- 

 missure and the pineal body. Crossing the centre of the ventricle, 

 from one optic thalamus to the other, is a band of grey matter 

 the middle or soft commissure ; and care should be taken that this 

 is not torn through in exposing the ventricle. 



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

 iii the choroid plexus. 



