iNERVOUS SYSTEM. 321 



transversely a firm medullary chord, which is extended in a 

 curved direction, on either side, into the substance of the corpus 

 striatum : this forms a band of connexion between the hemi- 

 spheres, and takes the name of commissura inferior cerebri. 

 Superiorly, above the commissura mollis, is another medullary 

 chord, similar to the one last mentioned, but shorter and smaller, 

 which is the commissura superior cerebri. 



Foramen. — Just over the commissura inferior, between it and 

 the commissura mollis, between and in front of the optic thalami, 

 under the arch of the fornix, is a triangular hole, called ihe fora- 

 men commune inferius. Now, since the lateral ventricles com- 

 municate underneath the arch of the fornix, it is evident that 

 both of them must open into this foramen ; and since this fora- 

 men leads into the third ventricle, it follows that this cavity 

 and the lateral sinuses must reciprocally communicate through its 

 medium. 



Between the commissura mollis and the commissura superior 

 is another foramen, which would also be one of like communica- 

 tion were it not covered and closed by the velum interpositum, 

 which is here spread over the surfaces of the thalami, and tra- 

 versed by the vessels of the choroid plexuses 



The third ventricle, as it is called, is nothing more than the 

 fissure existing between the thalami, in consequence of their 

 partial divergence, below and behind the commissura mollis, 

 in the medium line of the brain. The floor or back part of this 

 narrow oblong cavity is constituted of the crura cerebri. 



[nfundibu/uni. — Leading from the inferior part of this ven- 

 tricle, is the infundibulum, a passage small at its commence- 

 ment, but gradually enlarging to its termination, which takes 

 place in a blind extremity, in front of the corpus albicantium, at 

 the pituitary gland. Superiorly, under the superior commissure, 

 is a passage leading from this cavity into the fourth ventricle 

 (situated within the cerebellum), the 4ter a tertio ad quartuni 

 ventriculum. 



Pineal Gland. — Over the third ventricle, deeply lodged be- 

 tween the summits of the thalami, above and rather before the 

 superior commissure, we find the pineal gland — a little, conoid, 

 greyish body, marked by a slight depression along its middle ; 

 enveloped in pia mater, derived from the vessels of the choroid 

 plexus; and attached by that membrane to the thalami, and to 

 the tubercula quadrigemina, which are placed behind it. From 

 its base or inferior part proceed two \\i\\Q peduncles or foot-stalks, 

 which run along the top of the superior commissure, and are 

 implanted into the crura cerebri. The structure of this little 

 organ remains unknown. It is possessed of considerable firmness, 

 T t 



