THE ISTHMUS. 



775 



commissure — a thin fasciculus of transverse fibres placed in advance of the corpora 

 quadrigemina, above the entrance to the aqueduct of Sylvius (or iter a tertio ad 

 quartum ventrindum), the extremities of which are lost in the substance of the 

 thalami optici (Fig. 427, 9). The anterior common foramen, also designated the 

 foramen of Monro (and iter ad infundibulum) (Fig. 429, 14), is the medium of 

 communication between the middle and lateral ventricles, and affords a passage 

 to the vascular cord which unites the two choroid plexuses. It is pierced in 

 front of the grey commissure, beneath the summit of the fornix, the two pillars 

 of which concur to circumscribe it, and between which is seen the anterior ivhite 

 commissure. This is a small band of white transverse fibres, analogous to that 

 which constitutes the posterior commissure, but thicker, and passing in front of 

 the anterior pillars of the fornix, its extremities entering and becoming lost in 

 the corpus striatum on each side. 



The posterior extremity of the middle ventricle — narrower than the anterior, 

 and placed on a more elevated 



plane— is continuous with the aque- F'i? ■iS? 



duct of Sylvius, the entrance of 

 which (Fig. 427, 10) is beneath the 

 posterior commissure, towards the 

 common foramen. 



The anterior extremity, more 

 dilated than the posterior, is situ- 

 ated immediately above the optic 

 chiasma, and is only separated from 

 the bottom of the great interlobular 

 fissure of the brain by a small and 

 very thin grey lamina attached to 

 that chiasma, and for this reason 

 named by writers the grey root of 

 the optic nerves. This lamina 

 (lamina cinerea) is readily seen when 

 the optic commissure is turned back 

 on the pituitary gland ; it is suf- 

 ficient to traverse this to enter the 

 middle ventricle. 



The ependymis, which forms the 

 wall of the central canal of the 

 spinal cord, also lines the walls of 



this cavity ; through the aqueduct of Sylvius, it is prolonged into the posterior 

 (or fourth) ventricle ; by the foramen of Monro it passes into the lateral 

 ventricles, and thence into the spaces in the middle of the olfactory lobes. 



TRANSVERSE SECTION OF THE BRAIN AT THE 

 POSTERIOR COMMON FORAMEN. 



White substance of the hemisphere, or centrum 

 ovale of Vieussens ; 2, 2, grey substance forming 

 the external layer of the convolutions; 3, section 

 of the corpus callosum ; 4, 4, interior of the lateral 

 ventricles ; 5, section of the great vena Galeni ; 

 6, 6, cerebral peduncles ; 7, 7, section of the 

 isthmus ; 8, posterior common foramen ; 9, pos- 

 terior white commissure ; 10, entrance to the 

 aqueduct of Sylvius. 



2. Aqueduct of Sylvius (Fig. 429, 6). 



This is a longitudinal median canal, passing beneath the corpora quadrige- 

 mina, and above the crura cerebri. 



It is prismatic in shape ; its anterior extremity communicates with the 

 middle ventricle, and the posterior opens below the valve of Vieussens into the 

 fourth ventricle. 



