562 THE NERVOUS SYSTEM. . "q 
the sinus floor. Nor does the bone escape. As the Pacchionian bodies enlarge 
they cause absorption of the cranial wall, and small pits are hollowed out on its 
deep surface for their reception. It must be clearly understood, however, that in 
such cases the Pacchionian body is separated from the bone by the following — 
(1) A continuation round the Pacchionian body of the subdural space ; (2). the 
thinned floor of the parasinoidal sinus; (3) the lumen of the sinus ; and (4) the 
ereatly thinned upper wall of the sinus. 
The Pacchionian bodies have a special function to perform. ‘Through them tluid can pass 
from the subarachnoid space into the venous sinuses with which they stand in connexion. 
Whenever the pressure of blood in the sinuses is lower than that of the fluid in the subarachnoid 
space and the ventricles of the brain, the cerebro-spinal fluid filtrates through the Pacchionian 
bodies into the blood sinuses. This is not the only way that subarachnoid fluid may obtain 
exit. The subarachnoid space is carried outwards for a short distance on the nerves in connexion 
with thei arachnoidal sheaths, and communicates with the lymph channels of the nerves. 
This connexion is more complete in the case of the olfactory, the optic, and the auditory nerves, 
than in other nerves. A very free communication between the subarachnoid space and the 
lymphatics of the nasal mucous membrane is said to exist. 
THE PIA MATER. 
The pia mater forms the immediate investment of the brain and cord. It is < 
delicate and very vascular membrane. 
Pia mater encephali.— The pia mater which covers the brain is finer and 
more delicate than that which clothes the spinal cord. It follows closely all the 
inequalities on the surface of the brain, and in the case of the cerebrum it dips 
into each sulcus in the form of a fold which lnes it completely. On the cerebellum 
the relation is not so intimate; 
it is only into. the larger fissures 
Genu of corpus that it penetrates in the form of 
; callosum 
folds. 
( Petey srs The larger blood-vessels of the 
Saptumeneram brain lie in the subarachnoid 
Lo oe Gidnre nanleas space. The finer twigs ramify in 
the pia mater before they proceed 
into the substance of the brain. 
As they enter they carry with 
them sheaths derived from the 
pia mater. When a portion of 
aN Optic thalamus the membrane is raised from 
the surface of the encephalon, 
| numerous fine processes are with- 
Aes Caen drawn from the cerebral surface. 
| | Choroid plexus of These are the blood-vessels with 
their sheaths, and they give the 
deep surface of the pia mater a 
rough and flocculent appearance. 
a2 As the pia mater is carried over 
Fornix 
—Anterior pillar of fornix 
-7—Vein of corpus striatum 
Posterior pillar of fornix > Vit. 
Ti (ahder sueEGa) once the lower part of the roof or 
posterior wall of the fourth ven- 
fy Body of fomnie Chitown tricle of the brain it receives the 
| backwards) name of the tela choroidea inferior, 
and it 1S In connexion with this 
t portion of the pia mater that the 
FIG. epee SS ECON TO SHOW THE VELUM INTERPOSITUM, choroid plexuses of that cavity are 
AND THE PARTS IN IMMEDIATE RELATION TO IT. r : 
developed. The tela choroidea 
superior or velum interpositum is a fold of pia mater which is invaginated into the 
brain, so that it comes to lie over the third ventricle and project in the shape of 
choroid plexuses into the lateral ventricles. This invagination requires special 
notice. 
The velum interpositum (tela choroidea superior) is a double layer or fold of pia 
tis 
