THALAMI OPTICI. THIRD VENTRICLE. 417 



The choroid plexuses communicate, and the veins of the corpora striata 

 pass through this opening. 



The lateral thin edges of the fornix are continuous posteriorly with 

 the concave border of the hippocampus major at each side, and form 

 the narrow white band called corpus fimbriatum (posterior crus of the 

 fornix). In the middle line the fornix is continuous with the corpus 

 callosum, and at each side with the hippocampus major and minor. 

 Upon the under surface of the fornix towards its posterior part, some 

 transverse lines are seen passing between the diverging corpora fim- 

 briata : this appearance is termed the lyra (corpus psalloides), from a 

 fancied resemblance to the strings of a harp. 



The fornix may now be removed by dividing it across anteriorly, 

 and turning it backwards, at the same time separating its lateral con- 

 nections with the hippocampi. If the student examine its under sur- 

 face, he will perceive the lyra above described. 



Beneath the fornix is the velum interpositum, a duplicature of pia 

 mater introduced into the interior of the brain, through the transverse 

 fissure. The velum is continuous at each side with the choroid plexus, 

 and contains in its inferior layer, two large veins (the venct Galeni) which 

 receive the blood from the corpora striata and choroid plexuses, and 

 terminate posteriorly, after uniting into a single trunk, in the straight 

 sinus. Upon the under surface of the velum interpositum are two 

 fringe- like bodies, which project into the third ventricle. These are 

 the choroid plexuses of the third ventricle; posteriorly these fringes en- 

 close the pineal gland. 



If the velum interpositum be raised and turned back, an operation 

 which must be conducted with care, particularly at its posterior part 

 where it invests the pineal gland, the thalami optici and the cavity of 

 the third ventricle will be brought into view. 



THALAMI OPTICI. The thalami optici are two oblong, square-shaped 

 bodies, of a white colour superficially, inserted between the two diverg- 

 ing portions of the corpora striata. In the middle line a fissure exists 

 between them, which is called the third ventricle. Posteriorly and in- 

 feriorly, they form the superior wall of the descending cornu, and pre- 

 sent two rounded elevations called corpus geniculatum externum and 

 internum. The corpus geniculatum externum is the larger of the two, 

 and of a greyish colour ; it is the principal origin of the optic nerve. 

 Anteriorly, the thalami are connected with the corpora albicantia by 

 means of two white bands, which appear to originate in the white 

 substance uniting the thalami to the corpora striata. Externally they 

 are in relation with the corpora striata and hemispheres. In their in- 

 terior the thalami are composed of white fibres mixed with grey sub- 

 stance. They are essentially the inferior ganglia of the cerebrum. 



THIRD VENTRICLE. The third ventricle is the fissure between 

 the two thalami optici. It is bounded above by the under surface of 

 the velum interpositum, from which are suspended the choroid plexuses 

 of the third ventricle. Its/oor is formed by the grey substance of the 

 anterior termination of the corpus callosum, called lamina cinerea, 



2 E 



