THALAMI OPTICI THIRD VENTRICLE. 



oeive the blood from the corpora striata and choroid plexuses, and termi* 

 nate 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 enclose 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 r . 



THALAMI OPTICI. The thalami optici are t\vo oblong, square-shaped 

 bodies, of a white colour superficially, inserted between the two diverging 

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

 them, which is called the third ventricle. Posteriorly and inferiorly, they 

 form the superior wall of the descending cornu, and present 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 con- 

 nected with the corpora albicantia by means of two white bands, which 

 appear to originate in the white substance uniting the thalami to the cor- 

 pora striata. Externally they are in relation with the corpora striata and 

 hemispheres. In their interior the thalami are compiled of white fibres 

 mixed with grey substance. They are essentially the inferior ganglia of 

 tJie 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 floor is formed by the grey substance of the anterior termi- 

 nation of the corpus callosum, called lamina cinerea, the tuber cinereum, 

 corpora albicantia, and locus perforatus. Laterally it is bounded by the 

 thalami optici ; anteiiorly by the anterior commissure and crura of the 

 fornix ; and posteriorly by the posterior commissure and the iter a tertio 

 ad quartum ventriculum. The third ventricle is crossed by three com- 

 missures, anterior, middle, and posterior ; and between these are two 

 spaces, called foramen commune anterius and foramen commime posterius. 



The interior commissure is a small rounded w 7 hite cord, which enters 

 the corpus striatum at either side, and spreads out in the substance of the 

 hemispheres; the middle , or soft commissure consists of grey matter, 

 which is continuous with the grey lining of the ventricle, it connects the 

 adjacent sides of the thalami optici ; the posterior commissure, smaller than 

 the anterior, is a rounded white cord, connecting the two thalami optici 

 posteriorly. 



The space between the anterior arid middle commissure is called the 

 foramen commune anterius, and is that to which Monro has given his 

 name (foramen of Monro). It is the medium of communication between 



foramen commune posterius. 1 3. The corpora quadrigemina, upon which is seen rest- 

 ing the pineal gland, 14. 15. The iter a tertio ad quartum ventriculum, or aqueduct of 

 Sylvius. 1G. The fourth ventricle. 17. The pons Varolii, through which are seen pass- 

 ing the diverging fibres of the corpora pyramidalia. 18. The crus cerebri of the left 

 side, with the third nerve arising from it. 19. The tuber cinereum, from which pro- 

 jects the infundibulum, having the pituitary gland appended to its extremity. 20. One 

 of the optic nerves. 21. The left olfactory nerve terminating anteriorly in a rounded 



uUlb. 



