366 NERVOUS SYSTEM. 



usual, and in its middle a fissure which separates almost com-' 

 pletely its two halves. 



As the fornix is fitted to the upper surface of the optic tha- 

 iami, it is of course concave below and convex above, or re- 

 sembles a triangular arch resting upon its three points or angles. 

 Owing to some misunderstanding of the orignal Greek work 

 *x>.lhs, which, according to the interpretation of Sabatier, 

 means a vault, and thereby expresses the whole body, anato- 

 mists, with the exception of him, have generally supposed the 

 striated under surface of the fornix to be meant by it, and 

 have called the surface Lyra, in which mistake one has followed 

 another. 



The Septum Lucidum is a partition placed vertically in the- 

 middle line of the brain, and extends from the corpus callosum 

 above to the fornix below. It is of a triangular shape, but ir* 

 regularly so, being much broader before than it is behind, and 

 having its edges so incurvated as to fit the bodies against which 

 it is applied. 



The septum lucidum is formed by two laminae placed side to 

 side, but not adhering to each other, and leaving, therefore, an 

 interval between them, called the Ventriculus Septi, or the fifth 

 ventricle. Each of these lamince consists of two layers; the 

 internal is medullary substance, continuous with that of the cor- 

 pus callosum and of the fornix; and the external is a layer of 

 cinefitious substance. The cavity is about an inch and a half 

 long by a line wide, and is narrower in the middle than at either 

 extremity. It is lined by a delicate serous membrane, which 

 becomes manifest when the halitus that naturally covers its sur- 

 face is accumulated into a body of water. It is generally sup- 

 posed to be insulated or completely shut up, yet occasionally it 

 has been found elongated in front, towards the space between 

 the anterior commissure and the anterior crura fornicis, and to 

 communicate there with the third ventricle.* 



The Pineal Gland (Glandula \<Pi?iealis, Conarium). is placed 

 beneath the posterior margin of the fornix, upon the superior of 

 the tubercula quadi igemina, or the nates. It is a.n oblong conoi* 



* J, F. Mcc'.icl. 



