ANATOMY. 
ject. In young foetuses it is softand almost 
fluid : it becomes of a more solid consis- 
tence in increasing age, and is firmest in old 
persons. 
The dura mater is a very firm and com- 
pact membrane, adhering closely by vessels 
and fibres to the internal surface of the cia- 
nium. It therefore to be regarded as the 
periosteum of the internal table of the cra- 
nium, as well as a membrane for supporting 
and investing the brain. It is described by 
anatomists as consisting of two layers, inti- 
mately connected in general, but separated 
from each other at particular parts, so as to 
leave vacancies between them called sinuses, 
into which the veins of the brain pour their 
blood. The chief of these are, the supe- 
rior longitudinal, the two lateral, and the 
torcular herophili. There are besides some 
smaller ones, as the inferior longitudinal, 
the cavernous, the circular, the superior, 
and the anterior petrosal. They all termi- 
nate ultimately in the lateral sinus, which, 
quitting the cranium, takes the name of in- 
ternal jugular vein. 
On the upper part of the dura mater 
some small eminences are observed, arising 
from clusters of white granular bodies, situ- 
ated between this membrane and the pia 
mater ; they are the glandute Pacchioni ; 
and fill the pits which may be observed in 
the skull-cap. The ramifications of the spi- 
nous artery, which is the chief nutrient ves- 
sel of the dura mater, are very conspicuous 
on each side of the head The inner sur- 
face of the dura mater is smooth and shin- 
ing, and has no connection with the pia 
mater, except where veins pass from tire 
latter membrane to the sinuses. 
The processes, which the dura mater 
forms, for' separating and supporting the 
different parts of the brain, are, 1 . the 
falx cerebri; 2. tentorium cerebelli; 3. 
falx cerebelli. 
The two membranes which immediately 
invest the brain, were considered as one, 
and called the pia mater, until a more mi- 
nute investigation had shewn that it could 
be divided into two layers. The outer one 
is called tunica arachnoidea. This is spread 
over the visible surface of the brain, is of a 
pale white colour, yet in some degree 
transparent, very thin, and devoid of evi- 
dent vessels. It is seen most evidently, 
where it passes between the two lobes of the 
cerebellum, and’ about the middle of the 
basis cerebri : in other parts, it adheres so 
intimately to the pia mater, that, the dis- 
tinction can scarcely be demonstrated. 
The pia mater every where covers the 
external surface of the brain, and therefore 
sends processes into all the convolutions of 
this organ. It is extremely vascular, and a 
great portion of the blood, which the brain 
receives, is spread out upon its surface in 
minute vessels. The outer surface is tole- 
rably smooth ; the inner universally villous, 
from the torn orifices of innumerable ves- 
sels, which entered the substance of the 
brain. 
The surface of the brain appears convo- 
luted, so as to resemble the windings of the 
small intestines. These convolutions do 
not in general penetrate more than one inch, 
or an inch and a half, into the substance. 
The contents of the cranium are divided 
into cerebrum, cerebellum, and medulla 
oblohgata. 
The cerebrum is the upper, and by far 
the largest portion : it occupies all the su- 
perior part of the vaulted cavity of the 
skull, and rests below on the tentorium, the 
petrous portions of the temporal bones, the 
sphenoid ate, and the orbits. Its upper 
surface presents a regularly convex oval, 
narrower in front than behind. It is divided 
"into a right and left hemisphere by a deep 
longitudinal fissure, into which the falx ce- 
rebri descends. Each hemisphere is divided 
into two lobes by means of the fissura 
magna Sylvii. This fissure commences at 
the basis of the brain, opposite to the lesser 
ala of the sphenoid bone ; the anterior lobe 
is that portion of the hemisphere situated 
in front of the fissure ; and the posterior 
lobe is the division placed behind. 
The hemispheres of the cerebrum are 
united together at about two inches and a 
half from the surface of the brain by means 
of a medullary body, called corpus callosum. 
This is about three inches in 'length and 
three quarters of an inch in breadth. 
As there are no distinguishable parts in 
the upper portions of the hemispheres of the 
cerebrum, it is customary to pare all these 
away in dissection^ nearly to the level of 
the corpus callosum, in order that we may 
be able more easily to open, And more parti- 
cularly to examine, certain cavities, which 
are situated at the sides of that body, and 
are called the lateral ventricles. 
On making a section of the brain, we 
perceive that it is composed of two sub- 
stances ; an exterior one, which is of a grey 
colour, and an interior one, which is white. 
These are simply termed the cineritious and 
white' substances, or substantia cinerea et 
alba ; or, from the former surrounding the 
