250 THE ANATOMY OF THE HORSE. 



ciu-A-es downwards, forwards, and inwards, terminating at the base of the 

 hemisphere in the uncinate convolution. The ventricles are lined by a 

 ciliated epithelium, whicli is continuous through the foramen of Monro 

 with the lining of the 3rd ventricle. 



The Septum Lucidum is a thin, translucent partition between the two 

 lateral ventricles. It is broadest in front, where it is attached to the 

 rostiTim, or reflected part of the corpus callosum. Its upper edge is 

 attached to the coi-pus callosum, and its lower edge to the fornix ; and 

 posteriori}- these edges meet at an acute angle. The septum consists 

 of white matter in its centre, with a layer of grey matter on each side. 

 In man it contains a small isolated cavity — the 5th ventricle. 



The Fornix, or arch, is a mesially placed white band, consisting of a 

 central part, or body, and two paii-s of processes, or pillars. The body 

 is flattened above and below, and broadest behind. Its upper face is 

 adherent posteriorly to the corpus callosum, but in front it dips down 

 and leaves beneath the forepart of the corpus callosum a space occupied 

 by the septiim lucidum. The under surface of the body rests on the 

 velum interpositum, and at its anterior extremity arches over the 

 foramen of Monro. The anterior ]nnars of the fornix are two white 

 cords which descend in front of the foramen of Monro, being separated 

 by a slight interval. Reaching the base of the brain, they turn on 

 themselves, forming thus the corpus albicans, and they then enter the 

 optic tlialamus. The posterior pillars are broader and flatter, and not 

 so well defined. Each in part bestows its substance on the surface of the 

 hippocampus, and in part it descends along the anterior edge of that 

 body as the taenia hipjiocampi, or corpus fimhriatum. 



The Corpus Striatum (Plates 35 and 36). This is the large grey 

 body already noticed in the body and anterior comu of the lateral 

 ventricle. In shape it is pyriform, having its broad end directed 

 forwards and inwards, and its tapering end backwards and outwards to 

 the roof of the descending comu. The body comprises two masses of 

 grey matter, separated from each other by intermediate white fibres 

 which curve upwards and outwards from the cerebral crus. The upper 

 mass of grey matter — termed the nncleus caudatus — is that which pro- 

 jects into the lateral ventricle. The lower mass — the nucleus lenticu- 

 laris — lies above the quaclrilatercd space already seen at the base of the 

 hemisphere between the diverging olfactory tracts. 



The T.ENLv Sbmicircularis (Plate 36) is a narrow white band that 

 extends between the corpus striatum and the optic thalamus. (Its 

 relation to the thalamus will be better seen in the next stage of the dis- 

 section.) 



The Hippocampus is the curved eminence already noticed in the body 

 and descending horn of the lateral venti'icle. It rests on the optic 

 thalamus, from which it is separated by the velum interpositum. The 



