NERVOUS CENTRES. (Human Anatomy. Tue Menrnces.) 
The surface of each choroid plexus presents 
many slight projections or folds resembling 
villi, in which are contained loops and plexi- 
form anastomoses of minute vessels, very si- 
milar to the arrangement of the vessels of the 
villous processes of the chorion of the ovum, 
or those of the tufts of the placenta. These 
vessels are surrounded by an epithelium which 
has much the appearance of that of serous 
‘membranes. From the great number of these 
vessels and from the delicate nature of the 
ithelial covering which surrounds them, it is 
lain that the choroid plexuses are well suited 
either for the purpose of pouring out fluid or 
of absorbing it. 
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Side view of villi of the choroid plexus of the lateral 
- ventricle in the brain of a Goose, to show the 
_ disposition of the bloodvessels. Not to obscure the 
view of the bloodvessels, the edge of the epithelium 
shown. 
h only has been 
ia 4, epithelium; 5, bloodvessels. 
. (After Valentin. ) 
vy ~ 
_ The epithelium may be best seen by examin- 
ing the edge of afold. It becomes very distinct 
when acted upon by acetic acid. As its particles 
“are very delicate and consist only of a single 
layer, they are easily detached. The cells of 
epithelium are most of them six-sided, and 
contain a clear nucleus, or several minute gra- 
nules. Valentin states that cilia may be seen 
playing upon this surface, especially in the 
yo. Ihave observed the peculiar punc- 
tiform or spiniform formations to which he 
alludes, which look like the remains of former 
vibratile cilia. 
Velum interpositum. (Toile Choroidienne, 
Vieg d’Azyr.)—The choroid plexuses are con- 
nected to each other by the velum interpositum, 
which is a triangular fold of pia mater that 
passes in at the transverse fissure between the 
upper surface of the tubercula quadrigemina 
and the posterior reflected portion of the corpus 
callosum. This process is continuous with the 
ia mater of the inferior surface of the posterior 
lobes of the brain, and with that of the superior 
surface of the cerebellum, and it therefore con- 
sists of two lamine; as it passes forwards, it 
sends downwards a little process which em- 
braces the pineal body ; it forms the roof of the 
third ventricle, being interposed between that 
cavity and the fornix, (hence its name,) and 
at its sides as well as its apex its continuity 
635 
with the choroid plexuses may be readily de- 
monstrated. At its anterior extremity it(_orre- 
sponds to the foramen commune anterius. The 
velum interpositum is best exposed in the dis- 
section from above downwards by removing 
carefully in succession the corpus callosum and 
the fornix. In raising the velum itself, in order 
to disclose the cavity of the third ventricle, it 
is necessary to be very careful, as from the in- 
timate connexion which the pineal body has 
with it towards its base, that body may be 
readily disturbed from its position. 
Choroid plexuses of the fourth ventricle.— 
The choroid plexuses of the fourth ventricle 
are two small processes of pia mater united 
along the median line, presenting the same 
villous character as those of the lateral ventri- 
cles. These folds seem as if they had been 
tia up into the fourth ventricle by the 
ower lamine of the inferior vermiform process. 
Their position may be best seen by opening the 
fourth ventricle from above, where they will be 
found lying on each side of that portion of the 
median lobe of the cerebellum which stops up 
the inferior extremity of the fourth ventricle. 
These plexuses are in every respect similar, as 
far as regards structure, to the larger ones 
which are found in the lateral ventricles, and, 
like them, exhibit a delicate epithelium upon 
their surface. Upon the centre of each epithe- 
lium cell Valentin states that a pigment cor- 
puscule is deposited. ( Fig. 365.) 
Fig. 365. 
A highly magnified villus of the choroid plexus of 
human cerebellum. ( After Valentin. ) 
a, the villus; 6, the epithelium cells; c, the 
nuclei. 
These internal processes of the pia mater 
contain minute crystalline formations, a kind 
of very fine sand, which, however, is not con- 
stantly present in all brains. 
The grains are deposited in the meshes of the 
vascular plexuses. Sometimes they accumulate 
in masses so as to be visible to the naked eye 
or easily recognized by the touch. In general, 
however, they are microscopic, in form glo- 
bular, and connect themselves with the minute 
vascular ramifications like little bunches of 
grapes. They are found principally in the 
choroid plexuses of the lateral ventricles, and 
in that portion of the velum  interpositum 
which embraces the pineal body. In the for- 
mer they are most numerous at that part which 
was called by the Wenzels glomus, where the 
choroid plexus turns up from the inferior cornu 
into the horizontal portion of the lateral ven- 
tricle.* As regards chemical composition this 
* See Van Ghert de plexubus choroideis, 
Utrecht, 1837 ; Valentin, in Soemmering Anat., and 
