THE SEPTUM LUCIDUM. THE FORXIX. 545 



manner the eminentia collaterals corresponds with the posterior branch of 

 the fissure of Sylvius. 



As some discussion has recently taken place in this country with regard to the 

 value of the presence of the hippocampus minor in man, as a distinctive character of 

 the human brain, it may be well to mention that this structure has been found even 

 in the brains of quadrumana which do not belong to the highest group. In the 

 human subject the posterior cornu varies greatly in size, and the hippocampus minor 

 is still more variable in its development, being sometimes scarcely to be recognised, 

 and at others proportionally large. It is usually most developed where the posterior 

 cornu is longest ; but length of the posterior cornu, and prominence of the hip- 

 pocampus minor, by no means occur in proportion to the dimensions of the hemi- 

 sphere, but rather seem to be associated with thinness of both the medullary and 

 the cortical substance. 



The septum lucidum is a thin translucent partition, placed between the 

 two lateral ventricles. It extends vertically between the corpus callosum 

 above, and the anterior part of the fornix below ; and, as the latter sinks 

 down in front away from the corpus callosum, the septum is deep before 

 and narrow behind. Anteriorly it lies in the hollow of the bend of the 

 corpus callosum, in front of the fornix. 



The septum lucidum is double, being composed of two distinct laminse, 

 having an interval between them, which contains fluid and is lined 

 by an epitheliated membrane. This is the fifth ventricle, ventricle of the 

 septum, or Sylvian ventricle. 



Each of the laminse of the septum which form the sides of the fifth ven- 

 tricle, consists of an internal layer of white substance and an external layer 

 of grey matter. 



In the human embryo, and also in some animals, the cavity of this ventricle com- 

 municates with that of the third ventricle in front and below : but in the adult human 

 brain it forms a separate and insulated cavity. Tarin described a small fissure in it 

 between the pillars of the fornix ; but this is unusual. In disease it is sometimes 

 distended with fluid. 



The fornix is an arched sheet of white longitudinal fibres, which appears 

 partly in the floor of both lateral ventricles. It consists of two lateral 

 halves, which are separated from each other in front and behind, but 

 between those points are joined together in the mesial plane. The two 

 parts in front form the anterior pillars of the fornix ; the middle conjoined 

 part is named the body ; and the hind parts, which are again separated from 

 each other, form the posterior pillars. 



The body of the fornix is triangular in shape, being broad and flattened 

 behind, where it is connected with the under surface of the corpus callosum, 

 and narrower in front as it dips down to leave that body, the space 

 between them being filled up by the septum lucidum. Its lateral edges 

 are in contact with the choroid plexuses, and its under surface rests upon 

 the velum interpositum. 



The anterior crura or pillars of the fornix, cylindrical in form, descend, 

 slightly apart from each other, through a quantity of grey matter on the 

 sides of the third ventricle, between the corpora striata ; and, curving back- 

 wards as they descend, reach the corpora albicantia. There each crus turns 

 upon itself, making a twisted loop which forms the white portion of the 

 corpus albicans of its own side, and ascends to enter the substance of the 

 optic thalamus. These crura are connected with the peduncles of the pineal 

 gland, and with the tsenia semicircularis, as will be afterwards described. 



