PLEXUS CHOROIDES. 385 



is above it at that place ; but the under surface is attached 

 throughout to the parts on which it lies, by the aforesaid mem- 

 brane. 



The two posterior angles of the fornix form what are called 

 the crura, and they terminate in the following way. 



The surfaces of the inferior portions of the lateral ventricles 

 are not uniformly concave ; but at the bottom of each there is 

 a prominent body, which begins where this portion of the 

 cavity winds Outwards and forward, and continues its whole 

 extent. This prominence has a curved form, and is marked by 

 transverse indentations towards its extremity; hence it has 

 been termed the hippocampus, or cornu ammonis. 



A similar prominence, but smaller, and without the trans- 

 verse indentations, is to be found in the posterior portion of 

 the ventricle : this has, also, been called hippocampus ; but the 

 terms major and minor are applied to distinguish them. 



The posterior angles of the fornix terminate in the large 

 hippocampi ; and the margin or thin edge of the two anterior 

 sides of the fornix, is continued to form an edge to the hippo- 

 campus ; and is called the tcenia hippocampi, or corpus fim- 

 briatum. 



The word fornix was the ancient name of a vault or arch ; 

 and, from its supposed resemblance to an arch, this part has 

 been called by that name. 



When the fornix is raised up, which must be done by divi- 

 ding it at the anterior angle, and detaching it from the thalami 

 nervorum opticorum, the thalami by dissecting off the velum in- 

 terpositum, are brought fairly into view, and appear like oval 

 bodies placed parallel to each other. They adhere slightly at 

 their upper surfaces, and, when separated, a fissure appears 

 between them, which is the third ventricle. At the upper 

 and front part of this third ventricle, near its commencement, 

 before the anterior crura of the fornix, and very near them, is 

 a white chord, like a nerve, which passes across the ventricle, 

 and can be traced to some distance on each side of the medul- 

 lary matter of the brain. 



This chord is called the anterior commissure of the brain, 

 VOL. ii. 33 



