372 



THALAMI OPTICI. 



verse fissure. The velum is connected at each side with the choroid 

 plexus, and contains within its two layers, in the middle line, two 

 large veins, the venae. Galeni, which receive the blood from the 

 ventricles, and terminate posteriorly 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. 



Fig. 125 * 



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. 



The Thalami optici are two rounded oblong 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 



* The mesial surface of a longitudinal section of the brain. The incision has been 

 carried along the middle line ; between the two hemispheres of the cerebrum, and 

 through the middle of the cerebellum and medulla oblongata. 1. The inner surface of 

 the left hemisphere. 2. The divided surface of the cerebellum, showing the arbor vitee. 

 3. The medulla oblongata. 4. The corpus callosum, rounded before to terminate in 

 the base of the brain ; and behind, to become continuous with 5, the fornix. 6. One 

 of the crura of the fornix descending to 7, one of the corpora albicantia. 8. The 

 septum lucidum. 9. The velum interpositum, communicating with the pia mater of the 

 convolutions through the fissure of Bichat. 10. Section of the middle commissure 

 situated in the third ventricle. 11. Section of the anterior commissure. 12. Section 

 of the posterior commissure; the commissure is somewhat above and to the left of the 

 number. The interspace between 10 and 11 is the foramen commune anterius, in 

 which the crus of the fornix (6) is situated. The interspace between 10 and 12 is the 

 foramen commune posterius. 13. The corpora quadrigemina, upon which is seen 

 resting (he pineal gland, 14. 15. The iter e tertio ad quartum ventriculum. 16. The 

 fourth ventricle. 17. The pons Varolii, through which are seen passing 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 projects 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 bulb. 



