636 
sabulous matter consists chiefly of phos- 
phate of lime with a small proportion of phos- 
phate of magnesia, a trace of carbonate of lime, 
and a small quantity of animal matter. 
The pia mater adheres a closely to the 
surface of the brain, coming for the most part 
into contact with grey matter. When a portion 
of it is raised carefully in a fresh brain, num- 
berless extremely minute bloodvessels are seen 
Passing from it into the cerebral substance. 
ese are the principal nutrient vessels of the 
brain. On its outside the pia mater adheres 
partially to the arachnoid membrane. At those 
points which correspond to the convex portions 
of the convolutions the adhesion of arachnoid 
to pia mater is close; but at other places the 
latter membrane separates completely from the 
former. 
The pia mater of the brain differs from that 
of the spinal cord in its great delicacy and 
tenuity ; it wants the strength and density of the 
latter membrane. This 1s owing to its being 
com almost entirely of extremely minute 
and delicate bloodvessels, whilst the spinal 
membrane consists chiefly of white fibrous 
tissue. The bloodvessels of the former are in- 
finitely more numerous than those of the latter, 
and the reason of this probably is that the 
cerebral membrane is chiefly in contact with 
grey matter, which requires a great quantity of 
blood, but the spinal membrane immediately 
embraces white matter, which is much less 
vascular. 
It is important, in a pathological point of 
view, to notice that this membrane is the me- 
dium of nutrition, not merely to the nervous 
matter of the brain and cord, but also to the 
arachnoid membrane which is immediately 
adherent to it, and to which it bears the same 
relative position as the sub-serous areolar tissues 
elsewhere to their respective serous membranes, 
Hence the difficulty, if not the impossibility, 
of adopting distinctions which systematic wri- 
ters endeavour to make out between arachnitis 
and superficial inflammation of the brain. It 
is physically impossible that there shall be 
arachnitis without serious disturbance of an 
inflammatory kind in the circulation of the 
pia mater, nor can this exist without affecting 
the superficial layers of the grey matter of the 
convolutions. It may, therefore, be confidently 
affirmed that arachnitis, when affecting that 
portion of the arachnoid membrane which co- 
vers the hemispheres of the brain, is synony- 
mous with inflammation of the superficial 
layers of the grey matter of the convolutions. 
Whatever be the point of departure, it seems 
impossible that inflammation of the one can 
exist without a similar and equal affection of 
the other. And thus we may explain the ap- 
ntly anomalous statement of authors that 
inflammation of the arachnoid should give rise 
to a more violent train of ew than deep- 
seated inflammation of the brain. The real 
difference is, not between membranous and 
Bergmann, iiber die innern Organisation des 
Gehirns. The last author states that he has seen 
the sandy deposit excessive in connexion with 
mental derangement. 
NERVOUS SYSTEM. (Nervous Centres. Tat Mentnces.) 
cerebral inflammation, but between an inflan 
matory affection of the superficial grey matte 
of the convolutions, the great source and seat 
of the physiological activity of the brain, am 
a similar morbid action of the more centra 
white substance, the function of which is ina 
certain sense subservient to that of the super- 
ficial grey matter. — 
Of the arachnoid membrane.—This met 
brane is intermediate to those already describ 
We have preferred giving the description of 
last, because to understand it demands | 
acquaintance with the details of both the 
membranes. 7 
The arachnoid is a great serous mem 
pervading the entire cranio-spinal cavity. | 
— layer adheres intimately and insepar 
ly to the inner surface of the dura mater bo 
cranial and spinal, and its visceral q 
attached to the outer surface of the pia matet 
In point of structure and general dispositi 
the arachnoid membrane resembles other 
rous membranes, so much as to render it ine 
pedient to enter into any minute comparis 
of them. It will only be necessary to refer 
such peculiarities of arrangement as may 4 
from the anatomical characters of the nery 
centres with which it is connected. 
Spinal arachnoid —The serous che 
the spinal arachnoid is best seen by e: 
a transverse section of the spinal cord ar 
membranes. If the section be made ac 
the interval between two sets of spinal ner 
the visceral and parietal layers of the m 
brane may be seen in contact with each oth 
the parietal layer closely attached to the ¢ 
mater, the visceral layer adherent to the 
mater of the spinal cord so loosely as to hi 
a considerable space between it and the o 
surface of that membrane. 2% 
Fig. 366. 
Transverse section of spinal cord and its 
between the fifth and sixth cervical 
( After Arnold, ) 
v, visceral layer of arachnoid membrane; 
arachnoid space; c, arachnoid cavity. _ 
We may here notice an im dis 
which the student of this ion of ar 
will do well to note particularly, e 
the space between the two layers of are 
membrane is the arachnoid bag or & 
which it is very rare for any fluid to ac 
late; and that that between the visceral 
the arachnoid and the pia mater is t 
arachnoid cavi'y, in which, as will be 
by-and-bye, a considerable quantity of 
exists in the natural state. a 
When the section is made on a level wit 
nerves as they emerge through the dura 
we may notice the manner in w 
noid membrane is prolonged upon 
in the form of a loose sheath, 
’ f 
ormir 
