NERVOUS CENTRES. (HUMAN ANATOMY. THE ENCEPHALON.) 



of white or fibrous matter ; and a fourth con- 

 sists of an extremely delicate membrane, pro- 

 bably covered by ciliated epithelium, which 

 lines the internal surface of the fifth ventricle. 



The fifth ventricle is closed at every point, 

 and has, therefore, no communication with the 

 lateral or other ventricles. It has been regarded 

 by some as resulting merely from the artificial 

 separation of the laminae of the septum luci- 

 dum. And it seems unlikely that in life, 

 during health, the surface of these laminae 

 should be otherwise than in contact, lubricated, 

 however, by a slight moisture exhaled by the 

 membrane. In a few rare cases fluid has been 

 known to accumulate in this cavity. 



In the fifth month of uterine life, according 

 toTiedemann, this ventricle communicates with 

 the third through a small space, situate be- 

 tween the anterior pillars of the fornix and 

 above the anterior commissure, and indeed it 

 may be looked upon as a portion of the latter 

 ventricle closed oft' by the formation of the fornix 

 and septum lucidum. 



The following parts are to be noticed in each 

 lateral ventricle : 1 . In the anterior horn, the 

 corpus striatum, a pear-shaped eminence, the 

 obtuse extremity of which is directed forwards 

 and inwards. Posteriorly thisbody is apparently 

 prolonged backwards into the inferior cornu of 

 the lateral ventricle by a long tapering process 

 which terminates there. 2. Internal and pos- 

 terior to the corpus striatum is the optic tha- 

 lamus, a gangliform body of a greyish colour, 

 but considerably paler than that last named. 

 3. These two bodies are separated from each 

 other by a superficial groove, in which lies a 

 delicate band of fibrous matter, the tteriia semi- 

 circularise which is covered by a lamina of 

 horny-looking matter, lamina cornea, the forma- 

 tion of which is attributed by some to a thick- 

 ening of the lining membrane of the ventricle 

 along this groove. 



The choroid plexus in a great degree covers 

 and conceals from view the optic thalamus. It 

 passes up from the descending cornu, and just 

 behind the septum lucidum and anterior pillars 

 of the fornix turns inwards to unite with its 

 fellow of the opposite side. On its inner side 

 it is slightly overlapped by the thin margin of 

 the horizontal portion of the fornix. 



In the posterior horn we observe, on its in- 

 ternal wall, a projection inwards of one of the 

 convolutions to which the name hippocampus 

 minor, or ergot, has been given. It is an in- 

 ternal convolution, covered by a layer of fibrous 

 matter derived from the fornix. It is traversed 

 by a deep sulcus, which may be exposed by 



cutting it across. 



The descending horn contains a remarkable 

 prominence, the hippocampus major, (also called 

 cornu Anutumis,) which projects into it from its 

 inferior wall, and follows the curve of the horn. 

 It likewise may be regarded as an internal con- 

 volution, and is covered by a layer of fibrous 

 matter derived from the fornix, which overlaps 

 the concavity of the hippocampus by a thin 

 margin, called corpus Jimbriatum. Beneath this 

 is a peculiar disposition of grey matter con- 

 nected with the hippocampus, to which the name 



675 



fascia dentata has been given. The commence- 

 ment of the choroid plexus is found in this horn. 



The anterior extremity of the descending horn 

 of the lateral ventricle corresponds with the 

 posterior extremity of the fissure of Sylvius. 

 It is closed, not by nervous matter, but simply 

 by the reflection of the membrane of the ven- 

 tricle on the choroid plexus. This is the only 

 provision against the escape of fluid from the 

 ventricle. It seems highly probable, as we 

 have already intimated, that there may be a 

 communication at this situation, as well as 

 at the fourth ventricle, between the fluid of 

 the ventricles and that of the sub-arachnoid 

 cavity by endosmose and exosmose. And 

 the delicacy of the barrier which is opposed to 

 the escape of fluid from the ventricle explains 

 the occurrence of sanguineous effusions at the 

 base of the brain from the rupture of vessels 

 within the ventricle. 



Postponing the more minute description of 

 the parts found in the lateral ventricle, as above 

 enumerated, we proceed with the examination 

 of those which are brought into view beneath 

 the corpus callosum. 



The corpus callosum, which we have seen to 

 consist of bundles of transverse fibres, passes 

 directly from one hemisphere to the other. At 

 its anterior and posterior extremity it is folded 

 downwards, so as to connect those parts of the 

 hemispheres which lie on a plane inferior to 

 the lateral ventricles. Its anterior reflected 

 portion, therefore, contributes to form the floor 

 of the anterior horn, and the posterior one 

 mingles with the fibres of the inner wall of the 

 posterior horn. This disposition of the corpus 

 callosum is best seen on a vertical section of 

 the brain, which shows the vaulted form of this 

 body. The greater abruptness of reflection of 

 its posterior than of its anterior extremity, how- 

 ever, impairs in a great decree this character. 



Of the fornix. We have seen that the an- 

 terior reflection of the corpus callosum is oc- 

 cupied along the median plane by the vertical 

 septum lucidum. This septum rests posteriorly 

 upon the apex of a horizontal stratum of fibrous 

 matter which forms part of a series of fibres 

 called the Fornix or Vault. It is inconvenient 

 to change names which have long been in use, 

 more especially when there is no very certain 

 scientific foundation for the adoption of a new 

 one ; otherwise the term antero-posterior com- 

 missure, which is suggested by the direction and 

 the extensive connection of its fibres, might be 

 appropriately assigned to it. 



The principal portion or body of the fornix lies 

 immediately beneath the three posterior fourths 

 of the corpus callosum. By cutting this body 

 across just at the posterior extremity of the 

 septum lucidum, and dissecting the anterior 

 segment forwards, and the posterior one back- 

 wards, its horizontal portion is exposed. In 

 this dissection it is found that the latter portion 

 of the corpus callosum is intimately adherent 

 to the fornix. So close indeed is this adhesion 

 that the separation is always attended with 

 injury to the fornix. The deep-seated fibres of 

 the corpus callosum seem to unite the lateral 

 halves of the fornix. 



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