634 
fibrous tissue, which cannot confer elasticity. 
And if a portion removed from the cord be 
stretched, it will be found to possess very little 
elasticity ; but if the cord be held up by the 
filiform prolongation, and a slight jerking move- 
ment be communicated to it, it may be made 
to dance about as if by the elastic reaction of the 
filliform process. The movement which may 
be thus produced is very well calculated to 
deceive, and Dr. Macartney must have founded 
his opinion upon that experiment alone, omitting 
to try the effect of stretching a detached portion 
of the process. The fact is that when the cord is 
suspended in this way, the pia mater becomes 
stretched, and its anterior and rior por- 
tions are approximated and the cord flattened ; 
when it is raised with a jerk, this tension of 
the pia mater is diminished, and the cord re- 
turns to its previous form until it falls again, 
stretches the pia mater, and becomes once 
more flattened, producing a degree of reaction 
which favours its elevation, but which alone 
would be insufficient for that purpose. Thus 
it appears that the elastic reaction, which Dr. 
Macartney attributed to the filiform process, is 
in reality due to the compression and conse- 
quent flattening of the cord by the tension of 
the pia mater. It should be stated, further, 
that this process is not formed of pia mater 
alone, but also of a continuation of the liga- 
mentum denticulatum on each side to be 
described by-and-bye. 
The pia mater is abundantly supplied by 
bloodvessels, many of which are extremely 
tortuous. These vessels are derived from the 
anterior avd posterior spinal arteries. Along 
the anterior surface of the spinal cord in front 
of the anterior median fissure there is a narrow 
band of fibrous tissue which is stretched across 
this fissure like a bridge, and occupies its 
whole length. No such arrangement exists on 
the posterior surface. 
e pia mater of the spinal cord esses 
considerable strength and density. e ner- 
vous matter may by pressure be squeezed out 
of it, leaving a hollow cylindrical membrane, 
or it may be dissolved out by the action of 
liquor potasse. In the quite recent state, 
while the cord is as yet firm, the pia mater 
may be readily dissected off, its adhesion to 
the cord being through the medium of nu- 
merous exceedingly minute capillary vessels. 
On its exterior the pia mater adheres to the 
visceral layer of the arachnoid membrane by 
means of a loose fibrous tissue. 
Pia mater of the brain.—In tracing the pia 
mater of the spinal cord upwards, it will be 
found gradually to become much thinner and 
more delicate as it passes from the medulla ob- 
longata to the hemispheres of the cerebellum 
and cerebrum. In connexion with these latter 
it becomes of extreme tenuity, and owes 
its physical tenacity chiefly to the intimate con- 
nexion of the visceral layer of the arachnoid 
membrane with it. The cerebral pia mater is al- 
most exclusively composed of numerous ramifi- 
cations of minute vessels which are accompanied 
by white fibrous tissue in small quantity. These 
vessels divide and subdivide to the last degree 
NERVOUS SYSTEM. (Nervous Centres. Tue Mentnoes.) 
of minuteness, and are admirable objects for 
examining the structure of capillary vessels. 
The pia mater adheres closely to the whole 
surface of the brain, cerebellum, and connect- 
ing parts, and numberless vessels pass from it 
into the nervous substance in contact with it. 
On the surface of the brain it dips down into th 
sulci or furrows between the convolutions, an 
adheres to the superficial grey matter. Whe 
ever there is a depression or fissure of t 
brain, the pia mater is found dipping into } 
It likewise sinks into the fissures 
lamine of the cerebellum. 
We shall obtain, however, a very inadequat 
notion of the extent of the pia mater, 
confine our examination of it to the exterior 
the brain and cerebellum. At certain situation 
this membrane is continued into the caviti¢ 
or ventricles of these organs, where it doubt 
fulfils some office connected with the supp 
and nutrition of certain parts of them. Th 
situations are four in number, as follow: ¢ 
each side, the fissure between the crus cerel 
and the middle lobe of the brain, behind, 
transverse fissure between the cerebellum < 
cerebrum, and, lastly, the inferior extremity 
the fourth ventricle. 
Choroid plexuses of the lateral ventricles 
These are apparently folded processes © 
ia mater which enter the inferior part of 
teral ventricles on each side, and are conti 
upwards and forwards to their middle portic 
where they become continuous with each 01 
in the foramen commune anterius, 
a middle process, the velum. Each cho 
plexus forms a somewhat cylindrical pre 
which, when traced from below upwards” 
from behind forwards, will be found to foll 
the direction of the lateral ventricle as far 
wards as the apex of the horizontal portic 
the fornix, gradually diminishing in thie! 
and assuming the character of a simple 
branous expansion. It projects freely in 
cavity of the ventricle, having no comm 
with the walls of that cavity excepting | f 
the margins of the fissure, at which itt 
where the membrane of the ventricle a 
to it, being probably reflected upom it, 
a, villus; 6, epithelium; ¢, nucleus 6 
lium. re 
. 
/ 
we 
Very numerous and tortuous blood 
contained in these processes, forming’ 
which has given name to the folds themsel 
