“THE PLA MATER. 563 
mater which intervenes between the body of the fornix which lies above it and the 
epithelial roof of the third ventricle, and the two optic thalami which lie below it. 
Between its two layers are blood - vessels, and some subarachnoidal trabecular 
tissue. In shape the velum interpositum is triangular, and the narrow anterior 
end or apex reaches forwards as far as the foramina of Monro. The base lies 
under the splenium of the corpus callosum, end here the two layers of the velum 
separate and become continuous with the investing pia mater on the surface of the 
brain by passing out through a cleft called the transverse fissure. 
Along each lateral margin the velum interpositum is bordered by the choroid 
plexus of the body of the lateral ventricle, which projects into the ventricular 
cavity from under cover of the free lateral margin of the fornix. It should be 
borne in mind that the epithelial lining of the ventricle gives a complete covering 
to the choroid plexus. Posteriorly the choroid plexus is continuous with the 
similar structure in the descending horn of the ventricle, whilst in front it narrows 
greatly, and becomes continuous across the mesial plane with the corresponding 
plexus of the opposite side, 
behind the epithelial layer 
which lines the foramen of 
Monro. From this median 
junction two much smaller 4.) 
choroid plexuses run back- nucleus 
wards on the under surface Position.of tanta 
of the velum interpositum, 7" 
and project downwards into 
Lateral ventricle 
S Se / Choroid plexus of lateral ventricle 
> » 
x 
Velum inter- 
the third ventricle. These positum 
are the choroid plexuses of 
the third ventricle. Tenia thalami~ 
The most conspicuous 
blood-vessels in the velum = Choreid plexus Vent. TIL. 
interpositum are the two 
veins of Galen, which run Se 
backwards, one on either 
: : a Fic. 418. —DIaGRaMMaTIc CoRONAL SECTION through the optic 
side of the mesial plane. 
thalami, and the parts in immediate relation to them. ‘The inter- 
In front, each is formed at mediate part of the great transverse fissure holding the velum 
the apex of the fold by the interpositum is seen, and also the manner in which this fissure is 
shut out from the lateral ventricles by the epithelium which covers 
union of the vein of the the choroid plexus on each side. 
corpus striatum and a large 
vein issuing from the choroid plexus ; behind, they unite to form the vena magna 
Galeni, and this pours its blood into the anterior end of the straight sinus (Fig. 413, 
p: 557). 
The continuous cleft in the brain through which the velum interpositum and the choroid 
plexuses of the two descending horns of the lateral ventricles are introduced into the interior of 
the brain is sometimes called the transverse fissure. It consists of an upper intermediate part 
and two lateral parts. The former passes forwards between the corpus callosum and the fornix 
above and the roof of the third ventricle and the optic thalami below. It is limited on either 
side by the epithelial covering of the choroid plexuses which shuts out these structures from 
the cavity of the lateral ventricles. The lateral part is the choroidal fissure. This is continuous 
with the intermediate part, and has already been described in connexion with the descending 
horn of the lateral ventricle (p. 535). 
Pia mater spinalis—The pia mater of the cord is thicker and denser than 
that of the brain. This is largely due to the addition of an outside fibrous layer, 
in which the fibres run chiefly in the longitudinal direction. The pia mater is 
very firmly adherent to the surface of the cord, and in front it sends a fold into the 
antero-median fissure of the cord. The septum which occupies the postero-median 
fissure is likewise firmly attached to its deep surface. In front of the antero- 
median furrow of the cord the pia mater is thickened in the form of a longitudinal 
glistening band, termed the linea splendens, which runs along the whole length of 
the cord, and blends with the filam terminale below. The blood-vessels of the spinal 
cord lie between the two layers of the pia mater. 
The nerves which leave both the brain and cord receive closely-applied 
