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NERVOUS CENTRES. (Human Anatomy. Tur Encepuaton.) 
pf 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 lamine of the septum luci- 
_ dum. And it seems unlikely that in life, 
during health, the surface of these lamine 
should be otherwise than in contact, lubricated, 
however, by a slight moisture exhaled by the 
membrane. Ina fewrare cases fluid has been 
known to accumulate in this cavity. 
In the fifth month of uterine life, according 
_ to Tiedemann, 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 off by the formation of the fornix 
__ and septum lucidum. 
___ ‘The following parts are to be noticed in each 
Tateral ventricle :—1. In the anterior horn, the 
“corpus siriatum, a pear-shaped eminence, the 
_ obtuse extremity of which is directed forwards 
_ and inwards. Posteriorly this body 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- 
Zamus, 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 tenia semi- 
circularis, which is covered by a lamina of 
_ horny-looking matter, /amina cornea, the forma- 
ion 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 
i up from the descending cornu, and just 
behind the septum lucidum and anterior pillars 
of the fornix turns inwards to unite with its 
sllow of the opposite side. On its inner side 
it is slightly overlapped by the thin margin of 
the horizontal portion of the fornix. 
the posterior horn we observe, on its in- 
_ ternal wall, a projection inwards of one of the 
_ conyolutions to which the name hippocampus 
» or ergot, has been given. It isan in- 
mal convolution, covered by a layer of fibrous 
atter derived from the fornix. It is traversed 
by a deep suleus, which may be exposed by 
_ cutting it across. 
The descending horn contains a remarkable 
prominence, the hippocampus major, (also called 
. cornu Ammonis,) 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 
concavity of the hippocampus by a thin 
margin, called corpus fimbriatum. Beneath this 
is a peculiar disposition of grey matter con- 
nected with the hippocampus, to which the name 
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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 
porenier extremity of the fissure of Sylvius. 
t 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 degree 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 anteru-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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