758 DISSECTION OF THE BRAIX. 



the cavity on the right side, as much of the corpus callosum as 

 forms the roof of the space is to be removed. A part of the 

 ventricle extends down into the temporal lobe towards the base of the 

 brain ; and to open it, a cut is to be carried outwards and down- 

 wards, through the substance of the hemisphere, along the course 

 of the hollow ; and the best way to do this is to remove the parts 

 with a scalpel, piecemeal, carefully following the descending horn 

 of the ventricle until the parts are displayed as in fig. 275. 

 Brain con- VENTRICLES OF THE BRAIN. Five ventricles are described in the 

 ventricles : ^rain ; but four of them are subdivisions of one large central 

 cavity, and these are lined throughout by a thin membrane named 

 the ependyma, which is covered by epithelium, for the most part 

 ciliated. They are the two lateral ventricles, one in each cerebral 

 hemisphere, the third ventricle close to the base of the brain 

 between the two hemispheres, and the, fourth ventricle between the 

 cerebellum and the back of the pons and medulla oblongata (fig. 274). 

 tifth is The fifth ventricle is a small space between the layers of the septum 

 from others lucidum, and has not any lining of ependyma (fig. 275 b.) 

 Lateral The LATERAL VENTRICLE (fig. 275) is a narrow space which 



ventricle. extends nearly the whole length of the hemisphere, and sends a 

 process downwards into the temporal lobe. The cavities of the 

 two sides approach one another in front, where they are only 

 separated by the thin septum lucidum ; and below the hinder part 

 of that partition, each communicates with the third ventricle by an 

 aperture known as the foramen of Monro (fig. 274). At the back 

 there is a wider interval between them. The roof of the space is 

 formed in its whole extent by the fibres of the corpus callosum 

 passing outwards to the convolutions; in the floor are numerous 

 objects which will be enumerated in connection with the several 

 parts of the ventricle. 

 Subdivision. The ventricle consists of a central part or body, and three 



processes or cornua, anterior, posterior, and middle or descending. 

 Body; The body is beneath the parietal lobe of the hemisphere, and 



extends from the foramen of Monro to the splenium of the corpus 

 callosum. It is somewhat arched, with the convexity upwards, and 

 in its floor are seen the following parts, proceeding from without 

 objects in inwards (fig. 275): 1, the hinder portion of a pyriform mass of 

 grey matter forming a part of the corpus striatum (caudate 

 nucleus ; e), 2, a slender white band the tsenia semicircularis (/ ), 

 3, a narrow part of the optic thalamus ((/), 4, a vascular fringe of 

 the pia mater the choroid plexus (7i), and 5, a thin \vhite layer 

 the lateral part of the fornix (c). It is bounded internally for a 

 mesial limit, short distance in front by the hinder part of the septum lucidum (6), 

 and behind this by the meeting of the fornix and corpus callosum. 

 Anterior The anterior cornu projects forwards, with an inclination down- 



wards and outwards, into the frontal lobe. In the floor are the 

 boundaries, large anterior extremity (head) of the caudate nucleus and the 

 rostrum of the corpus callosum ; its anterior boundary is formed by 

 the genu of the latter body ; and internally it is separated from the 

 cavity of the opposite side by the septum lucidum. 



