416 FORNIX. 



the base of the brain. The two crura descend in a curved direction to 

 the base of the brain, embedded in grey substance in the lateral walls 

 of the third ventricle, and lying directly behind the anterior commis- 

 sure. At the base of the brain they make a sudden curve upon them- 

 selves and constitute the corpora albicantia, from which they may be 

 traced upwards to their origin in the thalami optici. Opening trans- 

 versely beneath these two crura, just as they are about to arch down- 

 wards, is the foramen of communication between the lateral and the 

 third ventricles, the foramen o/Monro; or, for umen commune anterius. 



Fig. 140.* 



* 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 cere- 

 brum, and through the middle of the cerebellum and medulla oblongata. 1 . 

 The inner surface of the left hemisphere. 2. The divided surface of the cere- 

 bellum, shewing the arbor vitse. 3. The medulla oblongata. 4. The corpus 

 callosum, curving downwards in front to terminate at the base of the brain ; 

 and rounded 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 sep- 

 tum 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 com- 

 missure. 12. Section of the posterior commissure ; the commissure is some- 

 what above and to the left of the numeral. The interspace between 10 and 11 

 is the foramen commune anterius, in which the crus of the fornix (6) is situ- 

 ated. The interspace between 10 and 12 is the foramen commune posterius. 

 13. The corpora quadrigemina, upon which is seen resting the pineal gland, 14. 

 15. The iter a tertio ad quartum ventriculum, or aqueduct of Sylvius. 16. The 

 fourth ventricle. 17- The pons Varolii, through which are seen passing the 

 diverging fibres of the corpora pyramidalia. 18. The crus cerebn 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 termi- 

 nating anteriorly in a rounded bulb. 



