THE HIPPOCAMPUS. 585 



lops closely the pineal gland, and is extended over the top of the 

 third ventricle, constituting the velum interpositum, and down 

 the middle cornua of the lateral ventricles, forming the plexus 

 choroides. At the bottom of the middle cornua, the plexus 

 choroides communicates with the pia rnater lining the fissure of 

 Sylvius. The ventricular serous or arachnoid membrane, is a 

 distinct membrane having no communication with that on the 

 outer surface of the brain. The so called posterior commissure 

 of the third ventricle, is formed merely by the bulging forwards 

 of the white fibres of the nates, in consequence of the cineritious 

 developments within. 



The Fornix, (inferior longitudinal commissure,) is extended 

 over the upper surface of the velum interpositum. It has the 

 shape of a vaulted roof, as its name imports, and forms the roof 

 of the third ventricle, and part of the lower surface or floor of 

 the two lateral. It arises by two rounded cords called the ante- 

 rior pillars or crura of the fornix, from the eminentice mammil- 

 lares (bulbce fornicis) two small rounded bodies that are found 

 at the basis of the brain, (see fig. 224,) medullary externally, 

 and cineritious within, and into which fibres extend both from 

 the ascending columns of the medulla oblongata, and from the 

 interior of the thalamus. 



The course of each of these cords, is first forward, then up- 

 ward, and then backward, through the grey matter lining the 

 lateral walls of the third ventricle. The crura thus form two 

 semicircles, concave posteriorly, that shortly unite on the median 

 line, the back surface of which is free and unattached, while the 

 front convex surface, receives fibres from the anterior lobes and 

 from the under part of the great transverse commissure or corpus 

 callosum, by which means, a thin delicate partition is formed, 

 between the lateral ventricles, called septum lucidum. The fornix 

 in its passage backwards and under the corpus callosum, to which 

 it is attached, spreads laterally, and at first is nearly of the width 

 of half an inch ; but afterwards becoming much wider, it de- 

 scends to be connected with the convolutions, first of the back 

 part, and afterwards of the inner and under part of the posterior 

 lobes. Two bands of white matter called the posterior pillars 

 or crura of the fornix, pass one on either side down into the mid- 



