NERVOUS SYSTEM. (Nervous Centres. Asnormat Anatomy.) 
The existence of serous fluid in the arach- 
noid cavity is of very rare occurrence. In some 
instances old adhesions of the two layers of 
arachnoid to each other circumscribe a space in 
which fluid accumulates. 
Blood is sometimes effused into the subarach- 
noid cavity. This is frequently the case in 
injuries of the head, the blood escaping from 
broken vessels of the pia’ mater. Sometimes 
the blood effused into either lateral ventricle 
will escape into the subarachnoid cavity, break- 
ing down the membrane of the ventricle. If an 
apoplexy occur near the surface of the brain, 
the laceration of the cerebral substance may 
extend quite to the surface, and the blood may 
pass through the pia mater into the subarach- 
noid space. 
In some instances we find blood in the 
cavity of the arachnoid (the arachnoid sac). 
The blood is either loose in the sac, or it is 
more or less closely connected with the inner 
surface of the membrane lining the dura mater. 
__. In arecent communication from Mr. Pres- 
_ cott Hewitt, published in the last volume of 
the Medico-Chirurgical Transactions, the prin- 
cipal facts relating to this subject have been 
_ collected and arranged in an interesting form. 
_ Mr. Hewitt describes these effusions of blood 
as existing in four forms’:— 
“1. The extravasated blood may be either 
liquid or coagulated ; if in the latter state, it 
may be in clots, or spread out in the shape of a 
thin membranous layer, covering a greater or 
less extent of the surface of the brain. 
“2. Sometimes the extravasation presents 
itself under the shape of a false membrane, 
possessing more or less of the original colour of 
_ the blood. 
_ 3. The blood may be fixed to the free 
_ surface of the arachnoid and there maintained 
G by a membrane, which to the naked eye pre- 
? pa all the characters of the serous membrane 
itself. 
* “4, The blood is frequently found enclosed 
ina complete cyst of various degrees of thick- 
ness, which may be removed unbroken from 
_ the cavity of the serous membrane. 
__ “The four divisions above referred to,” adds 
Mr. Hewitt, “‘ may be and often are combined 
" with each other, but in whatever state the extra- 
-vasated blood has been found, it has, in the 
‘Majority of cases, corresponded to the upper 
‘surface of the brain, and has been rarely met 
with in the cerebellar fossz.” 
It is impossible that these effusions of blood 
can have any other source but the minute 
bloodvessels of the pia mater or the dura mater, 
which becoming ruptured allow the blood to 
burst through the serous membrane by which 
they are covered. They occur mostly in per- 
‘sons of a scorbutic or hemorrhagic habit, or 
in whom the arteries have become brittle from 
abnormal deposits in them; and it is not im- 
probable that whilst the imperfect nutrition of 
the arteries is going on, the serous membrane 
itself suffers, becomes wasted, and therefore 
easily yields to the force of the blood as it 
escapes from the bloodvessels. - ‘ 
Pus is found in the subarachnoid cavity 
717 
where there has been inflammation of the pia 
mater and arachnoid, and more rarely in the 
arachnoid sac. 
Of the pia mater—This membrane being 
the vascular membrane of the brain, and con- 
taining the nutrient vessels as well of the sur- 
face of the brain as of the visceral layer of the 
arachnoid, is the seat of all those changes in 
the condition of the bloodvessels or of their 
contents, which give rise to, or are caused by, 
morbid states either of the nervous matter or 
of the serous membrane. 
All those changes which indicate hyperemia 
or anemia of the convolutions of the brain 
occur in the pia mater ; and the colour of this 
membrane will vary according to the quantity 
of blood contained in its bloodvessels. 
There are no definite signs which enable the 
anatomist to pronounce whether an hyperemia 
be of the active and inflammatory kind, or 
passive, and dependent on some cause remote 
from the brain itself, or even upon a post- 
mortem cause, unless it be accompanied with 
those undoubted products of the inflammatory 
process, pus or lymph. 
A highly injected state of the vessels of the 
pia mater will frequently be caused by the 
manner of the patient’s death. Where the 
respiratory actions have been laboured and dif- 
ficult prior to death, this is sure to occur: we 
find it also when death has been caused by 
asphyxia, however produced. 
In convulsive diseases the pia mater and the 
whole brain become highly injected more as a 
consequence of the impeded circulation caused 
by the struggles of the patient interfering with 
the due exercise of the respiratory movements, 
than as the cause of the convulsions. Indeed, 
there seem good grounds for believing that con- 
vulsions are more frequently caused by a de- 
ficient supply of blood to the brain than by a 
superabundant flow of it to that organ. 
he pia mater is the seat of the principal 
morbid deposits which affect the brain. Of 
these tubercle is among the most common ; 
it most frequently occurs on the surface of the 
convolutions; but it may be found wherever 
the pia mater exists, either in the interior or on 
the outside of the brain. It occurs less fre- 
quently on the pia mater of the cerebellum 
than in any other situation. 
The tubercular deposit in the pia mater com- 
mences by the developement of minute granu- 
lations of a grey, clear, or semi-transparent 
material. These are deposited close to each 
other over a greater or less surface, forming a 
group, and several such groups may be formed 
near each other. After a time this grey ma- 
terial is changed into a yellow granular matter, 
which is sometimes enclosed in a cyst. _ 
Tubercular matter originally deposited on 
the surface of the pia mater in the sulcus of a 
convolution may have the appearance as if it 
had been formed in the substance of the brain. 
The sulcus is obliterated, and the tubercle, 
enlarging towards the brain, becomes, in a 
short time, surrounded by cerebral matter. 
Sometimes tubercle deposited in some part 
of the pia mater excites inflammation in the pia 
