IXTEEXAL PAETS. 54S 



The hippocampus minor is not peculiar to the human brain, but has been 

 found in the brains of quadi-umana. 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 jiropor- 

 tionally large. It is usually most developed where the posterior cornu is longest ; 

 but the length of the posterior cornu, and prominence of the hippocami^us minor, 

 are by no means in proportion to the dimensions of the hemisphere, but rather 

 seem to be associated with thinness of both the medullary and the cortical 

 substance. 



The septiim Iticidum (fig. 378) 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, in form somewhat resem- 

 bling an obovate leaf. Anteriorly it lies in the hollow of the bend of 

 the corpus callosum, in fi-ont of the fornix. 



The septum lucidum is double, being composed of two distinct 

 lamintB, having an interval betw'een them, which contains fluid and is 

 lined by an epitheliated membrane. This is thejifth ventricle, ventricle 

 of the septum, or Sylvian ventricle. 



Each of the laminre of the septum which form the sides of the fifth 

 ventricle, consists of a layer of white substance on the middle, and a 

 layer of grey matter upon each surface. 



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

 communicates with that of the thhd 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 (fig. 379,/) is an arched band 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 l)0(li/ ; and the hind 

 parts, which, are again seuarated from each other, form the posterior 

 pillars. 



The lodij 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 space beneath the 

 velum interpositum, and between the two optic thalami, forms the third 

 ventricle, to be afterwards described. 



The anterior crura or ^jillars of the fornix (fig. 383, e), 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. 

 Curving backwards as they descend, they reach the corpora albicantia. 

 There each cms turns upon itself, making a twisted loop which forms 

 the wliite portion of the corpus albicans of its own side, and ascends 

 to enter the substance of the optic thalamus. These crura ai'e con- 

 nected with the peduncles of the pineal gland, and with the taenia 

 semicircularis, as will be afterwards described. 



