LATERAL VENTRICLES. 523 



serous fluid, which is sometimes, even in health, secreted in considerable quantity. 

 These cavities are two in number, one in each hemisphere, and they are separated 

 from each other by a vertical septum, the septum lucidum. 



Each lateral ventricle consists of a central cavity or body, and three smaller 

 cavities or cornua, which extend from it in different directions. The anterior 

 cornu curves forwards and outwards, into the substance of the anterior lobe. The 

 posterior cornu, called the digital cavity, curves backwards into the posterior lobe. 

 The middle cornu descends into the middle lobe. 



The central cavity or body of the lateral ventricle is triangular in form. It 

 is bounded, above, by the under surface of the corpus callosum, which forms the 

 roof of the cavity. Internally, is a vertical partition, the septum lucidum, which 

 separates it from the opposite ventricle, and connects the under surface of the 

 corpus callosum with the fornix. Its floor is formed by the following parts, 

 enumerated in their order of position, from before backwards, the corpus striatum, 

 taenia semicircularis, thalamus opticus, choroid plexus, corpus fimbriatum, and 

 fornix. 



The anterior cornu is triangular in form, passing outwards into the anterior 

 lobe, and curving round the anterior extremity of the corpus striatum. It is 

 bounded, above and in front, by the corpus callosum; behind, by the corpus 

 striatum. 



The posterior cornu or digital cavity curves backwards into the substance of 

 the posterior lobe, its direction being backwards and outwards, and then inwards. 

 On its floor is seen a longitudinal eminence, which corresponds with a deep sulcus 

 between two convolutions ; this is called the hippocampus minor. Between the 

 middle and posterior horns, a smooth eminence is observed, which varies consi- 

 derably in size in different subjects. It is called the eminentia collateralis. 



The corpus striatum has received its name from the striated appearance which 

 its section presents, from white fibres diverging through its substance. The intra- 

 ventricular portion is a large pear-shaped mass, of a gray color externally ; its 

 broad extremity is directed forwards, into the forepart of the body, and anterior 

 cornu of the lateral ventricle ; its narrow end is directed outwards and backwards, 

 being separated from its fellow by the thalami optici ; it is covered by the serous 

 lining of the cavity, and crossed by some veins of considerable size. The extra- 

 ventricular portion is imbedded in the white substance of the hemisphere. 



The taenia semicircularis is a narrow, whitish, semitransparent band of medul- 

 lary substance, situated in the depression between the corpus striatum and thala- 

 mus opticus. Anteriorly, it descends in connection with the anterior pillar of the 

 fornix ; behind, it is continued into the descending horn of the ventricle, where 

 it becomes lost. Its surface, especially at its forepart, is transparent, and dense 

 in structure, and was called by Tarinus the horny band. It consists of longitu- 

 dinal white fibres, the deepest of which run between the corpus striatum and 

 thalamus opticus. Beneath it is a large vein, the vena corporis striati, which 

 receives numerous smaller veins from the surface of the corpus striatum, and 

 thalamus opticus, and terminates in the venae Galeni. 



The choroid plexus is a highly vascular, fringe-like membrane, occupying the 

 margin of the fold of pia mater (velum interpositum), in the interior of the brain. 

 It extends, in a curved direction, across the floor of the lateral ventricle. In front, 

 where it is small and tapering, it communicates with the choroid plexus of the 

 opposite side, through a large oval aperture, the foramen of Monro. Posteriorly, 

 it descends into the middle horn of the lateral ventricle, where it joins with the 

 pia mater through the transverse fissure. In structure, it consists of minute and 

 highly vascular villous processes, the villi being covered by a single layer of 

 epithelium, composed of large round corpuscles, containing, besides a central 

 nucleus, a bright yellow spot. The arteries of the choroid plexus enter the ven- 

 tricle, at the descending cornu, and, after ramifying through its substance, send 

 branches into the substance of the brain. The veins of the choroid plexus ter- 

 minate in the venae Galeni. 



