NEUROLOGY. 115 



each lateral ventricle, we commence with the corpus striatum, a pear-shaped 

 eminence situated in the anterior horn, the obtuse extremity directed for- 

 wards and inwards. Posteriorly the body is apparently prolonged back- 

 wards into the inferior cornu of the lateral ventricle by a long tapering 

 process, which terminates there. Internal and posterior to the corpus 

 striatum is the optic thalamus, a gangliform body of greyish color. Tliese 

 two bodies are separated from each other by a superficial groove, in which 

 lies a delicate band of fibrous matter, the taenia semicircularis, which is 

 covered by a lamina of horny-looking matter, the lamina cornea. 



The choroid plexus is a fold of thin and very red vascular membrane 

 derived from the pia mater, which enters the inferior cornu by the great 

 cerebral fissure, and ascending, turns inwards just behind the septum luci- 

 dum and anterior pillars of the fornix, and unites with its fellow of the 

 opposite side, covering and nearly concealing from view the optic thala- 

 mus. 



In the posterior cornu, there is seen in its internal wall a projection 

 upwards of one of the convolutions, to which the name of hipjwcamjms 

 minor has been given. It is traversed by a deep furrow. In the descend- 

 ing horn is a remarkable prominence, the hippocampus major, projecting into 

 it from the inferior wall and following the curve of the horn. Beneath this 

 is a peculiar grey matter, connected with the hippocampus, known as fascia 

 dentata. A thin margin from the fornix which overlaps the hippocampus 

 is called the corjous ßmbriatiwi. 



The fornix is a white, fibrous, triangular arch or vault, convex above, situ- 

 ated horizontally beneath the corpus callosum and septum lucidum; it lies 

 on the velum interpositum and choroid plexuses, and over the third ventricle. 

 The base, posteriorly, arises by two long flat bands {the posterior pillars or 

 crura), one from either side, by three roots, from the hippocampus major and 

 minor and from the t;^nia hippocampi ; these crura unite to form the body 

 of the fornix, which bending forwards divides into two short round cords, 

 the anterior pillars of the fornix. The inferior surface of the fornix is marked 

 posteriorly with several fine oblique lines {li/ra or corpus psaUoides); although 

 described as single it is really double, and divisible throughout except 

 where the lyra unites opposite sides. It ma}^ be called the great inferior 

 commissure of the cei'ebrum, the corpus callosum constituting the great 

 superior commissure. The anterior pillars of the fornix bound a space in 

 front, through which the lateral ventricles communicate with each other 

 through \h.Q foramen commune anter ius. 



The third ventricle. If the fornix be divided transversely at about its 

 middle, and the segments reflected, the velum interpositum being removed, 

 a fissure known as the tinrd ventricle, and situated between the optic thalami, 

 will be exposed. Anteriorly and between the anterior pillars of the fornix 

 this fissure is limited by a band of white matter called the anterior commis- 

 sure. At its posterior extremity the third ventricle becomes much con- 

 tracted in all its dimensions, and is continuous with a canal {aqueductus 

 Sylvii) which leads to the fourth ventricle. The roof of the third ventricle 

 is formed by the velum interpositum. 



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