LATERAL VENTRICLES. 593 



striatum, tasnia semicircularis, thalamus opticus, choroid plexus, corpus firnbri- 

 aturn, 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. Be- 

 tween the middle and posterior horns a smooth eminence is observed, which 

 varies considerably in size in different subjects. It is called the eminentia 

 collateralis. 



The corpus slriatum has received its name from the striped appearance which 

 its section presents, in consequence of diverging white fibres being mixed with 

 the gray matter which forms the greater part of its substance. The intraven- 

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

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

 cornu of the lateral ventricle : its narrow end is directed outwards and back- 

 wards, 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 extraventricular portion is imbedded in the white substance of the hemisphere. 



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

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

 lamus 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 fore part, is transparent, and 

 dense in structure, and this was called by Tarinus the horny band. It consists 

 of longitudinal white fibres, the deepest of which run between the corpus 

 striatum and thalamus opticus. Beneath it is a large vein (vena corporis striali}, 

 which receives numerous smaller veins from the surface of the corpus striatum 

 and thalamus opticus, and joins the venae Galeni. 



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

 margin of the fold of pia mater (velum interposition}, 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 ventricle at the descending cornu, and, after ramifying through 

 its substance, send branches into the substance of the brain. The veins of the 

 choroid plexus terminate in the venae Galeni. 



The corpus fimbriatum (tsenia hippocampi] is a narrow, white, tape-like band, 

 situated immediately behind the choroid plexus. It is the lateral edge of the 

 posterior pillar of the fornix, and is attached along the inner border of the 

 hippocampus major as it descends into the middle horn of the lateral ventricle. 

 It may be traced as far as the pes hippocampi. 



The thalami optici and fornix will be described when more completely ex- 

 posed, in a later stage of the dissection of the brain. 



The middle cornu should now be exposed, throughout its entire extent, by introducing the 

 little finger gently into it, and cutting outwards, along the f.nger through the substance of the 

 hemisphere, which should be removed, to an extent sufficient to expose the entire cavity. 



The middle, or descending cornu, the largest of the three, traverses the middle 

 lobe of the brain, forming in its course a remarkable curve round the back of 

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