716 
When there has been a solution of continuity 
and a loss or removal of any portion of the 
cranium, the exposed surface of the dura mater 
1s apt to throw out a growth of granulations 
which constitute the fungus of the dura mater, 
analogous to that which sprouts from the sur- 
face of a similar fibrous membrane—the tunica 
albuginea of the testicle. In point of structure 
this fungoid growth is the same as the granula- 
tions on the surface of external ulcers. 
Effusion of blood, constituting a form of 
meningeal apoplexy, may occur on the external 
surface of the den mater separating it from the 
bone; or on its internal surface, dissecting 
away thearachnoid membrane from its adhesion 
to the dura mater. The former kind is mostly 
if not always traumatic, that is, resulting from 
' the application of violence to the exterior of the 
cranium. The latter kind is of extremely rare 
occurrence, and must be carefully distinguished 
from that variety of effusion into the arachnoid 
sac in which the effused blood appears to be 
covered by a serous membrane. This mem- 
brane, however, results from the condensation 
of the superficies of the clot by its friction 
against the parietal arachnoid, and it may be 
distinguished | from a true serous membrane by 
the absence of epithelium from its free surface. 
Effusions of either kind generally occur on 
some part of the surface of the cerebral hemi- 
spheres above the level of the petrous bone. 
The arachnoid membrane. — The arach- 
noid membrane is sometimes the seat of 
acute inflammation, and presents the same 
signs of that process as are met with in other 
serous membranes. The chief and, indeed, 
the only unequivocal sign of his condition 
as of recent occurrence is the exudation of 
lastie lymph upon the free surface of either or 
th layers of the membrane, with or without 
pus. This is attended with a highly injected 
state of the subjacent tissue (pia mater or dura 
mater, generally the former). The arachnoid 
itself, it will be remembered, contains no blood- 
vessels, but derives its nourishment from the 
vessels of the subserous tissue. Its apparent 
vascularity is due to its great tenuity and trans- 
parency, which allow the bloodvessels lying 
underneath to be seen through it as if they be- 
longed to the membrane itself. 
An opaque condition of the arachnoid, vary- 
ing both in degree and extent, is a very common 
appearance of this membrane, especially at the 
middle and advanced periods of life. This 
occurs sometimes in patches; at other times it 
is generally diffused over the whole membrane. 
It is most conspicuous on the convex surface 
of the brain, especially towards the great longi- 
tudinal fissure, and it is frequently associated 
with large and numerous Pacchionian bodies. 
It occurs, however, very commonly at the base, 
and frequently opposite the confluxes of the 
subarachnoid fluid. 
The opacity of the arachnoid is commonly 
attributed to a former acute inflammation of 
the membrane, or to a chronic inflammation 
going on up to the time of death. But this 
state of the membrane is of such frequent oc- 
currence, and is so often found in persons who ° 
NERVOUS SYSTEM. (Nervous Cexrres. Asyormat Anatomy.) } 
evinced no sign of important organic change 
during life, that it seems scarcely correct to — 
attribute it to such a cause. It is not meant 
to deny that previous inflammation or chroni¢ 
inflammation is capable of causing these 
opaque spots, but undoubtedly other causes 
may produce them as well. friction 0} 
two opposed surfaces may do it, and depo: 
upon the free surface of the membrane, an ab 
tered condition of the epithelium, may 
the same effect. Some recent mi 
examinations convince me that morbid deposit 
similar to those which are formed on the coa 
of arteries, may be found here, and occur ii 
those morbid states of the blood, and cons 
quently of the whole system, which are f 
able to the deposition of a morbid mate 
throughout the arterial system, or in the 
stance of viscera.* 
In confirmation of this view it may be st 
that opacities of the arachnoid are most com 
mon after the middle period of life, and th 
they are then almost uniformly associated wil 
a morbid state of the arteries of the brain ai 
of other portions of the arterial system. : 
Adhesion between the opposed surface 
the visceral and parietal layers of the arachno 
is (and the fact is curious) not of frequent ¢ 
currence, excepting at the convex border of 
falx cerebri, where the Pacchionian bodies a 
found. And the intrusion of these bodies in 
the longitudinal sinus frequently increases t 
closeness of that adhesion. cellular ¢ 
hesion so common in other serous membrat 
is rarely found in the arachnoid. f 
Small plates of cartilage or of bone are sor 
times found in connection with the arachnoi 
Their formation is generally the result of a 
vious morbid deposit which has subseque 
become converted into cartilage or bone. 
Effusion.—Effusions take place either | 
the subarachnoid or the arachnoid cavity. — 
existence of serum, in undue quentians 
former situation, must be looked on as am 1 
crease. in the fluid which naturally occup 
that space, and as we have already remé 
in a former part of this article, it takes pl 
consequence of the failure of the normal 
sure upon the vascular surface, and may 
be regarded as tending to preserve the fune 
of the brain than as producing an inju 
pressure upon it. Indeed I have always 1 
that in cases where an abnormal quart 
fluid existed in the subarachnoid cavi 
brain afforded no indication of its having 
rienced undue pressure previous to ¢ 
such cases the brain seems to contal 
blood than natural, and its anemia 1s 
obvious in the grey matter. n 
is hyperemia of the veins in the white 
of the hemispheres, as if the heart's foree, 
necessary to the venous circulation witl 
cranium than even to that of other part 
body, had been prevented from exertin 
influence through the capillaries upon 
blood in the veins. “7 
* It is probably to deposits of this kind thi 
kitansky sso wader “the title of “ Gallent 
Concretionen,” : 
- 
USUCUL 
OMe 
