550 THE CEREBRUM. 



the cerebellum, are seen the pineal body, the corpora quadrigemina, the 

 valve of Vieussens, and the processus a cerebello ad cerebrum. 



The THIRD VENTRICLE is a narrow longitudinal cleft placed between the 

 optic thalami, which bound it on its two sides. It is covered above by the 

 velum interpositum and the fornix. Beneath, its floor is formed by the 

 following parts, which have been already described as seen on the base of 

 the cerebrum ; viz., commencing from behind, the posterior perforated space, 

 the corpora albicantia, the tuber ciuereum and infundibulum, and the 

 lamina cinerea, the last of which also serves to close it in front, as high as 

 the anterior commissure. Behind, is the anterior opening of the aqueduct 

 of Sylvius. The cavity is crossed by three commissures, named from their 

 position, anterior, middle, and posterior. 



The middle or soft commissure is composed almost entirely of grey 

 matter, and connects the two thalami. It is variable in size, and some- 

 times wanting ; it is frequently torn across in examining the brain. 



The anterior commissure is a round bundle of white fibres, placed imme- 

 diately in front of the anterior pillars of the fornix, and crossing between the 

 corpora striata. It marks the anterior boundary of the ventricle ; its fibres 

 extend laterally through the corpora striata, a long way into the substance 

 of the cerebral hemispheres.. 



The posterior commissure, also white but of smaller size, is placed across 

 the back part of the ventricle, immediately before and below the pineal 

 body, with which and with the corpora quadrigemina it is intimately 

 connected. 



The corpora striata, situated in front and to the outer side of the optic 

 thalami, are two large ovoid masses of grey matter, the greater part of each of 

 which is embedded in the middle of the white substance of the hemisphere 

 of the brain, whilst a part comes to the surface in the body and 

 anterior cornu of the lateral ventricle. This intraventricular portion of 

 the corpus striatum is of a pyriform shape, its larger end being turned 

 forwards, and its narrow end being directed outwards and backwards, so 

 that the optic thalami of the two sides are received between the diverging 

 corpora striata. On cutting into it, there may be seen at some depth from 

 the surface white fibres, which are prolonged from the corresponding cerebral 

 peduncle, and give it the streaked appearance from which it has received its 

 name. 



The extraventricular portion of the corpora striata will be afterwards 

 described. 



Along the inner border of each corpus striatum, and in a depression 

 between it and the optic thalamus, is seen a narrow whitish semitrans- 

 parent band, named tcenia semicircularis, which continues backwards into 

 the descending cornu of the ventricle, where its connections have not been 

 determined with precision. In front it reaches the corresponding, anterior 

 pillar of the fornix, and descends in connection with that cord of white 

 substance. 



It is more transparent and firm on the surface, especially at its fore part : and 

 this superficial stratum has been named stria cornea. The taenia consists of longi- 

 tudinal white fibres, the deepest of which, running between the corpus striatum and 

 the thalamus, were named by Vieussens centrum geminum semicircular e. Beneath 

 it are one or two large veins, which receive those from the surface of the corpus 

 Btriatum and end in the veins of the choroid plexuses. 



The thalami optici (posterior ganglia of the brain) are of an oval shape, 



