TEE ISTHMUS. 683 



pierced in front of tlie grey commissure, beneath the summit of the fornix, 

 whose two jpillars concur to circumscribe it, and between which is seen the 

 anterior white commissure. This is a small band of white transverse fibres, 

 analogous to that which constitutes the posterior commissure, but stronger, 

 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 

 aqueduct of Sylvius, whose entrance (Fig. 325, 10) is beneath the pos- 

 terior commissure, towards the common foramen. 



The anterior extremity, more dilated than the posterior, is situated 

 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 writei-s the 

 ^rey root of the optic nerves. This lamina (lamina cinerea) is readily seen 

 when the optic commissure is turned down on the pituitary gland ; it is 

 sufficient to traverse this to enter the middle ventricle. 



The ependymis, which lines the central canal of the medulla spinalis, 

 also covers the walls of this cavity ; through the aqueduct of Sylvius, it is 

 prolonged into the posterior (or fourth) ventricle ; by the anterior common 

 foramen into the lateral ventricles, and thence into the spaces in the middle 

 of the olfactory lobes. 



2. Aqueduct of Sylvius. (Fig. 327, 6.) 



This is a longitudinal median canal passing beneath the corpora quad- 

 rigemina, and above the peduncles of the brain. 



Its anterior extremity communicates with the middle ventricle, and the 

 posterior opens below the valve of Vieussens into the cerebellar (or fourth) 

 ventricle. 



3. The Posterior or Gerebellar Ventricle. (Fig. 327, 5.) 

 This ventricle' (or sinus rhomhoidalis), situated beneath the cerebellum, 

 between its peduncles, and above the medulla oblongata and pons Varolii, 

 is a cavity elongated from before to behind, and almost entirely occupied 

 by the vermiform processes. 



Its superior wall is formed by these two processes, the valve of Vieussens, 

 and that of Eenault. The inferior, or floor of the cavity, is represented by 

 the excavation on the superior face of the medulla oblongata, and which is 

 prolonged in front, above the pons Varolii, to near the testes. 



The anterior extremity communicates with the aqueduct of Sylvius. 

 The posterior occupies the summit of the calamus scriptorius. 



STRUCTirRE OF THE ISTHMUS. 



The encephalic isthmus being only a prolongation of the spinal cord, 

 ought to resemble it in its structure ; and this is, in fact, what is observed, 

 particularly in its posterior part, the common features of their organisation 

 disappearing as we approach its anterior extremity. 



After what has been said as to the external conformation of the medulla 

 oblongata, we know that this organ presents, on each of its lateral halves, 

 traces of a division into three principal fasciculi : a superior, formed by 



~ • As the cerebellum concurs in the formation of this cavity it would perhaps be 

 better to defer its study until that organ has been described. 



