MIDDLE CORNTJ. 595 



minor, at the junction of the posterior with the descending cornu. Like the 

 hippocampi, it is formed of white matter corresponding to one of the sulci, 

 between two convolutions protruding into the cavity of the ventricle. 



The corpus fimbriatum is a continuation of the posterior pillar of the fornix, 

 prolonged, as already mentioned, from the central cavity of the lateral ventricle. 



Fascia dentata. On separating the inner border of the corpus fimbriatum from 

 the choroid plexus, and raising the edge of the former, a serrated band of gray 

 substance, the edge of the gray substance of the middle lobe, will be seen be- 

 neath it : this is the fascia dentata. Correctly speaking, it is placed external 

 to the cavity of the descending cornu. 



The transverse fissure is seen on separating the corpus fimbriatum from the 

 thalamus opticus. It is situated beneath the fornix, extending from the middle 

 line behind, downwards on either side, to the end of the descending cornu, 

 being bounded on one side by the fornix and the hemisphere, and on the other 

 by the thalamus opticus. Through this fissure the pia mater passes from the 

 exterior of the brain into the ventricles, to form the choroid plexuses. Where 

 the pia mater projects into the lateral ventricle, beneath the edge of the fornix, 

 it is covered by a prolongation of the lining membrane, which excludes it from 

 the cavity. 



The septum lucidum (Fig. 331) forms the internal boundary of the lateral ven- 

 tricle. It is a thin, semi-transparent septum, attached, above, to the under 

 surface of the corpus callosum ; below, to the anterior part of the fornix, and, 

 in front of this, to the prolonged portion of the corpus callosum. It is trian- 

 gular in form, broad in front, and narrow behind, its surfaces looking towards 

 the cavities of the ventricles. The septum consists of two laminse, separated 

 by a narrow interval, the fifth ventricle. 



Fifth Ventricle. Each lamina of the septum lucidum consists of an internal 

 layer of white substance, covered by the lining membrane of the fifth ventricle; 

 and an outer layer of gray matter, covered by the lining membrane of the late- 

 ral ventricle. The cavity of the fifth ventricle is lined by a serous membrane, 

 covered with epithelium, and contains fluid. In the foetus, and in some animals, 

 this cavity communicates, below, with the third ventricle; but in the adult, it 

 forms a separate cavity. In cases of serous effusion into the ventricles, the 

 septum is often found softened and partially broken down. 



The fifth ventricle may be exposed by cutting through the septum, and attached portion of 

 the corpus callosum, with scissors ; after examining which, the corpus callosum should be cut 

 across, towards its anterior part, and the two portions carefully dissected, the one forwards, the 

 other backwards, when the fornix will be exposed. 



The fornix (Figs. 331, 332) is a longitudinal lamella of white fibrous matter, 

 situated beneath the corpus callosum, with which it is continuous behind, but 

 separated from it in front by the septum lucidum. It may be described as con- 

 sisting of two symmetrical halves, one for either hemisphere. These two por- 

 tions are joined together in the middle line, where they form the body, but are 

 separated from one another in front and behind ; forming the anterior and pos- 

 terior crura. 



The body of the fornix is triangular ; narrow in front, broad behind. Its 

 upper surface is connected, in the median line, to the septum lucidum in front, 

 and the corpus callosum behind. Its under surface rests upon the velum inter- 

 positum, which separates it from the third ventricle, and the inner portion of 

 the optic thalami. Its lateral edges form, on each side, part of the floor of the 

 lateral ventricles, and are in contact with the choroid plexuses. 



The anterior crura arch downwards towards the base of the brain, separated 

 from each other by a narrow interval. They are composed of white fibres, 

 which descend through a quantity of gray matter in the lateral walls of the 

 third ventricle, and are placed immediately behind the anterior commissure. 

 At the base of the brain, the white fibres of each crus form a sudden curve upon 

 themselves, spread out and form the outer part of the corresponding corpus 



