384 FORNIX. 



A yellowish band called the tenia Tarini or horny band form- 

 ed by a thickening of the lining membrane of the ventricle over- 

 lays this vein. 



These surfaces constitute the bottom or floor of the first por- 

 tion of the ventricles, which is under the corpus callosum : upon 

 this floor is laid a thin lamen of the medullary matter, of a 

 triangular form, called the fornix, which covers the thalami 

 nervorum opticorum, and is attached to them by a membrane ; 

 so that when the ventricles are opened, the bottom appears to 

 consist of the corpora striata, and the fornix. 



The upper surface of roof of the ventricles is concave ; from 

 the middle of it, immediately under the raphe of the corpus 

 callosum, there proceeds downwards a partition of medullary 

 matter, which separates the two ventricles from each other. 

 This is called septum lucidum, from its being nearly transparent : 

 below, it adheres to the fornix, and anteriorly, it is continued 

 into the medullary matter, between the corpora striata. This 

 septum lucidum is formed of two lamina or plates, which are 

 separated from each other in the anterior portion of the septum, 

 and thus form a small cavity, which has communication with 

 the third ventricle of the brain. 



The fornix is not perfectly flat, but accommodated to the sur- 

 face of the thalami nervorum opticorum ; its under surface is 

 rather concave, and its upper surface convex. The anterior 

 angle passes down between the most anterior parts of the tha- 

 lami nervorum opticorum, and is divided into two small portions 

 called its crura, which can be traced some distance in that part 

 of the brain. 



The body of the fornix is attached to the surfaces of the 

 thalami nervorum opticorum on which it rests by a very 

 vascular membrane, that is spread over the thalami, and 

 called tela choroidea and velum interpositum. At the edges 

 of the fornix, there are many blood-vessels in the membrane, 

 arranged close to each other, which are called the plexus 

 choroides. 



The posterior side or edge of the triangular fornix termi- 

 nates in the corpus collosum, or the medullary matter which 



