J. Ryland (Joseph Ryland) Whitaker.

Anatomy of the brain and spinal cord online

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HEALTH SCIENCES STANDARD




HX64085651
QM451 .W581899 Anatomy of the brain



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HEALTH.



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ANATOMY



OK THE



BRAIN AND SPINAL CORD.



PRINTED BY

E. & S. LIVINGSTONE

4 MELBOURNE PLACE

EDINBURGH.



ANATOMY



OF THE



BRAIN AND SPINAL COKD>*



BY

J. RYLAND WHITAKER, B.A, M.B.(Lond.)

FELLOW OF THE EOTAL COLLEGE OF PHTSICIAXS, EDIXBUBGH;

LECTCKEE OX iXilOUT, SCEGEOXS' HALL, EDIXBUEGH, AXD LADIES' SCHOOL OF

1IEDICIXE, 1IIXTO HOrSE, EDIXBUBGH; EXA3IIXEE IX AXATOMT,

EOTAL COLLEGE OF PHTSICIAXS, EDIXBCBGH.



SIXTH THOUSAND .



EDINBURGH: E. & S. LIVINGSTONE.

1899.

("all eights eeserved]









ISV



TO



Sir JAMES ALEXANDER RUSSELL,

LL.D., M.A., M.B., B.Sc, F.R.C.F.E., F. R.S.Ed.,
Late Lord Provost of Edinburgh,

®Ijis littk fttatmal is respsrtfulhr gauicat^u

IN TOKEN OF MANY KINDNESSES.

THE AUTHOR.



PKEFACE TO THIED EDITION.



This little book, first written whilst a student at the
earnest request of students, has now reached a
third edition, and has become a favourite, perhaps
undeservedly, with those for whom it was originally
intended. Although fully conscious of its many defects,
I fear that my time, now so much occupied with teach-
ing, leaves me no chance of remodelling the book in
a way that more mature years and experience would
suggest. It must practically remain in the same garb,
however imperfect, in which it first appeared. This
the student, at least, will probably not regret.

In this edition my work has been confined to the
removal of the more glaring blemishes and errors, and
to the rewriting and bringing up to date some of the
sections. As the book is now much used by senior
students and by practitioners, a general summary, for
their special use, has been added at the end.

Though the actual number of pages has been slightly
increased, the matter has been shortened, for the
individual pages have been made smaller, more margin



viii PREFACE.

having been allowed, so that the book reads more
easily and more room has been obtained for extra
plates.

To Dr W. F. Harvey, and to my other demon-
strators and students who have so kindly, more than
once, read over the proofs, I cannot sufficiently express
my gratitude.

Finally, it remains for me to thank Messrs M 'Lagan
& Cumming for the trouble they have taken in the
reproduction of the plates, and the Messrs Livingstone
and staff for their uniform courtesy and excellent execu-
tion of the work.

Surgeons' Hall,
Edinburgh, 1st May 1899.



TABLE OF CONTENTS.



Dedication,
Preface, ...
List of Plates,



l'AGE
V

vii

xiii



SECTION I.

The Spinal Cord, its Membranes, and its Vessels,



Chapter I.



Membranes of the Spinal Cord,
I. Dttra Mater,
II. Pia Hater,
III. Arachnoid,

Cerebro-Spinal Fluid.



3
4
6
9
10



Spinal Vessels,



Chapter II.



11



Chapter III.




The Spinal Cord,


12


I. Fissures of the Cord,


14


II. Spinal Nerves,


16


III. White and Grey Matter of Cord,


17


1. White Matter,


IS


White Columns,


IS


White Commissure.


25


Structure of White Matter.


26


2. Grey Matter,


27


Cornua,


28


Grey Commissure,


i".'


Neuroglia,


30


Nervous Constituents,


31


Vesicular Columns,


32


IV. Deep Origins of Spinal Nerves, ...


37



TABLE OF CONTENTS.



SECTION II.

The Brain, its Membranes, and its Vessels,
Dissection to Expose Membranes,
Dissection to Remote Brain, ...



PAGE

43
43
44



Chapter I.



Membranes of Brain,




45


I. Dura Mater,




45


Falx Cerebri,




46


Falx Cerebelli,


... ...


47


Tentorium Cerebelli,


... 47


II. Pia Mater,




49


III. Arachnoid,


...


50


IV. Venous Sinuses,


Chapter II.


52


Vessels of the Brain,


...


57


I. Arteries, ...




57


II. Veins,




61



Chapter III.
Sub-divisions of the Brain,
I. Medulla Oblongata,

1. Fissures of Medulla,

2. White Matter — Areas of Medulla,

3. Grey Matter of Medulla,
Recapitulation,...

II. Pons Varolii,

1. White Matter of the Pons,
_'. Grey Matter of the Pons,

III. Cerebellum,

1. Lobes of Cerebellum, ...

2. Peduncles of Cerebellum,

3. Medullary Vela,

I. Grey Matter of Cerebellum,

Minute Structure of Grey Matter,
5. White Matter of Cerebellum,
The 4th Ventricle,



63
65
66

67

77
85

87
89
91

92
95
97
100
101
102
105
106



TABLE OF CONTENTS.



XI







PAGK


IV. Cerebkum,


...


113


I. Exterior of Cerebrum,


114


1.


Fissures of Cerebral Hemispheres,


114


2.


Lobes and Convolutions


116




Structure of Cerebral Cortex, ...


128


II. Base


of Cerebrum,


132


III. Interior of Cerebrum,


138


1.


Ventricles, ...


141




Lateral Ventricle,


142




The 3rd Ventricle, ..


146




The 5th Ventricle and Septum Lucidum,


147




Velum Interpositum,


148




Choroid Plexus,


149




Veins of Galen,


150




Great Transverse Fissure,


150


2.


Basal Ganglia,


151




Corpora Striata,


151




' Optic Thalami,


153




Corpora Geniculate, ...


155




Pineal Gland,


156


3.


White Strands,


157




Corpus Callosum,


157




Anterior Middle, and Posterior Com-






missures,


160




Fornix,


160




Internal and External Capsules,


163


IV. Crura Cerebri, &c,


165


1.


Crura,


165


2,


Corpora Quadrigemina,


169


3.


Aqueduct of Sylvius, ...


170


Origins of Cranial Nerves,


170


General Summary,


...


180



SECTION III.



Outlines of Development or —
I. The Spinal Cord,
II. The Brain,
Index,



190
191
193
199



LIST OF PLATES.



PLATE PAGE

I. — Fig. 1, General View of the Brain and Spinal Cord
(Hirschfeld and Leveille). Figs. 2 and 3, Mem-
branes of the Spinal Cord, ... ... ... 4

IJ. — Figs. 4, 5, and 6, Membranes of the Spinal Cord.
Fig. 7, Central Canal of Spinal Cord (Foster).
Fig. 8, Sections of Spinal Cord, ... ... 8

III. — Sections of Spinal Cord at various levels, ... 14

IV.— Fig. 13, Tracts of Spinal Cord. Fig. 14, Vesicular
Columns and Deep Origins of Spiual Nerves.
Fig. 15, Blood- Vessels, ... ... ... 22

V. — Various kinds of Nerve Cells of Spinal Cord, ... 28

VI.— Figs. 23, 24, 25, and 20, White Tracts of Spinal
Cord (Bristowe). Fig. 27, Pia Mater. Fig. 28,
White Nerve Matter of the Spinal Cord. Fig. 29,
Anterior Horn of the Spinal Cord, ... ... 32

VII.— Fig. 30, Different kinds of Cells of the Spinal Cord,
Neuroglia, &c. (Van Gehuchten). Figs. 31 and 32,
Collaterals (Edinger), ... ... ... ... 34

VIII.— Fig. 1, Collaterals. Fig. 2, Spiual Ganglion. Fig. 3,
Neuroglia Cells (Van Gehuchten). Fig. 4, Nerve
Cells,... ... 40

IX. — Fig. 33, Diagrammatic View of Parts of the Brain.

Fig. 34, Membranes of the Brain, ... ... 41

X. — Fig. 35, Membranes aud Vessels of the Brain
(H. & L.). Fig. 36, Venous Sinuses (Wilson's
Plates), ... ... ... ... ... 48

XL — Figs. 37 and 38, Venous Sinuses (Wilson), ... 52

XII.— Figs. 39 and 40, Blood- Vessels of the Brain (II. & L.), 58



xiv LIST OF PLATES.

PLATE PAGE

XIII.— Blood- Vessels of the Brain, ... ... ... 60

XIV. — Blood- Vessels of the Brain, and Areas of Motion and

of Sensation, ... ... ... ... ... 62

XV.— Views of the Medulla, ... ... ... ... 66

XVI. — Fig. 50, Diagram of Course of Fibres in the Spinal

Cord,... ... ... ... ... ... 70

XVII.— Sections of the Medulla, ... ... ... ... 78

XVIII. — Figs. 57 and 58, Nuclei in the 4th Ventricle

(Edinger), ... ... ... ... ... 82

XIX.— Figs. 59 and 60, Pons. Fig. 61, Crura Cerebri, ... 90

XX— Figs. 62 and 63 (H. & L.), Figures of Cerebellum, 94

XXI.— Figs. 64 and 65, Cerebellum. Fig. 66 (Edinger),

and Fig. 67, Structure of Cerebellum (Ferrier), 98

XXII. — Structure of Cerebellum (Edinger) ... ... 102

XXIIa.— Figs. 68 and 69, Structure of Cerebellum (Van

Gehuchten), ... ... ... ... ... 104

XXIII. — Convolutions of the Brain, ... ... ... 118

XXIV. — Convolutions of the Brain, ... ... ... 124

XXV.— Figs. 74 and 75, Relation of Fissures, &c, to Skull

and Scalp, ... ... ... ... ... 128

XXVI. — Figs. 76, 77, and 78, Structure of Cerebrum (Ramon y

Cajal), — ••• ... ... ... 130

XXVIL— Fig. 79, Base of the Brain, ... ... ... 134

XXVIII.— Fig. 80, Side View of the Brain, ... ... 135

XXIX. — Diagram of Ventricles, ... ... ... ... 133

XXX. — Diagram of Ventricles, ... ... ... 142

XXXI. — Diagram of Ventricles, ... ... ... ... 144

XXXII. — Velum Interpositum, &c. ... ... ... 145

XXXIII. — Diagram of Ventricles, ... ... ... 143

XXXIIIa.— Fig. 1, Velum Interpositum, Fig. 2, Fornix, &c, ... 150

XXXIV.— Capsules, k ... 154



LIST OF PLATES. xv

PLATE PAGE

XXX V— Fig. 87, Nuclei of 3rd and 4th Nerves (Edinger).
Fig. 88, Optic Nerves (Hill). Fig. 89, Fornix.
Fig. 90, Nucleus Oaudatus (Edinger). Fig. 91,
Fornix (own specimen, and Hill), ... ... 160

XXXVI.— Fig. 92, Blood- Vessels to Capsules. Fig. 93, Ver-
tical Section of the Cerebrum. Fig. 94, Diagram
of the 3rd Ventricle. Fig. 95, Fascia Dentata, &c, 164

XXXVII.— Fig. 96, Diagram of Tracts of Fibres (Hill). Fig. 97,
Tracts of Medulla (Edinger, Hill, Bruce). Fig. 98,
Inter-peduncular Space (Holden). Fig. 99, Eesti-
form Body (Edinger), ... ... ... ... 168

XXXVIII. — Nuclei of Origin of Cranial Nerves, ... ... 172

XXXIX. — Nuclei of Origin of Cranial Nerves, ... ... 176

XL.— Figs. 100 and 101, Tracts of Degeneration in the

Brain and Spinal Cord, ... ... ... 182

XL I. — Diagram of Course of Motor Fibres (Van Gehuchten), 184

XLII. — Diagram of Course of Sensory Fibres (Van Gehuchten), 186

XLIII. — Diagrams of Development of the Brain and Spinal
Cord (after Quain, Kolliker, Sclrsvalbe, and
Edinger), .. ... ... ... ... 192

XL IV. — Diagrams of the Vesicles of the Brain and the Parts

Developed therefrom (after Van Gehuchten), ... 194



ANATOMY

OF THE

BRAIN AND SPINAL CORD.



THE nervous system of man comprises two por-
tions, the Sympathetic and the Cerebrospinal,
The latter, the cerebro-spinal, has two parts: a central,
which consists of the brain and spinal cord, and a
peripheral, which consists of the several nerves of
the body.

NERVOUS SYSTEM.



1. Sympathetic. 2. Cerebro-spinal.

I

I I

(i) Central Part- (2) Peripheral Part-

Brain and Spinal Cord. The Nerves of the Body.

The Central Nervous System of man, the only
part treated of in this hook, is, as we have just
seen, made up of the Brain and Spinal Cord. Sur-
rounded by the bony wall of the cranium in the
one case, and of the spinal canal in the other, these
two parts of the great cerebro-spinal nervous system
are continuous with each other through the foramen
magnum. They are each enveloped in three distinct
membranes, the Meninges, which form additional pro-
tective sheaths around them, and help to support
them in their respective cavities.

B



2 THE SPINAL CORD

The Brain and Spinal Cord are, moreover, each
composed of two kinds of nervous substance, known
from their colour as the White and Grey Matter
respectively ; but with this difference in their
arrangement, namely, that in the Brain the grey
matter is situated chiefly on the outside, forming
the cortex ; whilst in the Spinal Cord the white
substance is external, and the orev matter forms
the central core or pith.

We shall describe, firstly, the Spinal Cord, with its
Vessels and Membranes; and, secondly, the Brain,
with its Vessels and Membranes.



AND ITS MEMBRANES.



SECTION I.



SPINAL CORD AND MEMBRANES.

Dissection. — To see the spinal cord and its membranes it will
be necessary to open the vertebral canal. To do this, remove the
muscles from the vertebral grooves, and saw through the laminae
of the vertebras on each side, close to their union with the pedicles,
being careful, especially in the dorsal region, to direct the edge of
the saw inwards. Carry the incision downwards as far as the lower
end of the sacral canal, and upwards as high in the neck as may
be convenient. Break through with the chisel any partially sawn
arches, cut through the various ligaments, and remove the pieces
of bones thus detached. Carefully clear away the connective
tissue, veins, and fat covering the outer aspect'of the dura mater,
snip through the articular processes with the bone forceps, and
dissect out one or more of the processes of the dura mater which
pass through the inter-vertebral foramina. Examine the outer
surface of the dural sheath, and then slit it open in its entire
length,

CHAPTER I.
MEMBRANES OF SPINAL CORD.

(Plates I. and II.)
These membranes are identical in many respects
with those of the Brain, and are therefore simi-
larly named. They are the Dura Mater, the Pia
Mater, and the Arachnoid. The Dura Mater is the
most external, the Pia Mater is in close contact
with the cord, and the Arachnoid forms a vertical,
tubular partition between the others, dividing the



4 THE SPINAL CORD

space between them into two — viz., the sub-dural
and the sub-arachnoid. Thus we speak of the sub-
dural space, meaning that between the dura mater
and the arachnoid, and of the sub-arachnoid, or that
between the arachnoid and the pia mater. For
convenience we shall describe (1) the Dura Mater;
(2) the Pia Mater ; (3) the Arachnoid.

I.— THE DURA MATER.

(Pigs. 2 to 6.)

The Dura Mater, the most external and the
strongest covering of the cord, is a firm fibrous mem-
brane, continuous at the foramen magnum with the
similar membrane which lines the cranial cavity.
Enclosed in the spinal canal, it does not, however,
form an endosteum to the bones, and in this respect
differs from the cranial dura mater. Its outer
surface has a shining, pearly-white appearance, and
is separated from the walls of the spinal canal by
loose areolar tissue with a little fat, and by a plexus,
of veins. Slender fibrous bands, especially at its
lower end, attach it closely to the posterior common
ligament of the vertebras. In extent the dural
sheath reaches from the foramen magnum to the
second or third piece of the sacrum, where it ends
in a conical cul-de-sac. As you will see upon open-
ing it, the cavity which it encloses is much longer
and wider than its contents — the spinal cord — for
this latter ends at the first or second lumbar
vertebra. Below the pointed termination of the
spinal marrow — conns meclullaris — the cavity of



Fir,. I



Fig. 2.



ki/perior
I gin us.



iteral *
inus.



Subdural space



Plate I.

A interior median f.



j 'erebrum



Subarachnoid
space



^Cerebellum



Spinal nerv



Medulla




. Dura mater.



irachno'ld.
Pia mater.



Spinal Cord



m terminale.



tda equina. M



Conns medullaris.



Spinal __ j
nerve roots. \



Ligament um
dentieulatum.. i



Epithelium.



Posterior median f.



Fig. 3.




ira mater.



achnoid.



Via mater.



Linea
plendens



AND ITS MEMBRANES. 5

the dural sheath is occupied by bundles of nerve
roots — cauda equina (Fig. 1) — in the midst of
which you will be able to pick out a slender, silvery-
looking thread, the filura terminate or central
ligament (Fig. 1, fil. ter.). Continuous with the
apex of the conus medullaris, this terminal filament
runs down the middle line amonsrst the nerve roots
to the lower end of the dural cavity. It there
pierces the dural sheath, receives an investment
from it, and passes along with this investment to be
attached to the back of the coccyx (filum dura?
matris spinalis, see page 13). Thus, we see that
the dura mater forms a very loose "covering to the
cord, and we find, moreover, that it has a greater
capacity in the neck and back than it has in the
loins. Smooth and glistening on its inner aspect,
it presents on each side a longitudinal series of
rounded openings arranged an pairs, one for each
spinal nerve root. These roots, as they pass out to
the inter-vertebral foramina, carry with them a
tubular prolongation of the dura mater which, in
part, ensheaths the spinal ganglia, and, in part,
blends with the periosteum of the neighbouring
bones (Figs. 4 and 6, page 8).

It is important to recollect that, although the
spinal cord itself ends at the spot indicated — viz.,
the second lumbar vertebra, the dura mater, the
arachnoid, and the cerebro-spinal fluid, extend as
far as the second piece of the sacrum, so that injuries
inflicted upon the spine as low down as this latter
point may cause death by inducing inflammation
of the meninges.



6 THE SPINAL CORD

Minute structure of the dura mater. —
Under the microscope the dura mater will be seen to
consist of white fibrous and elastic tissues, arranged
in longitudinal bands or lamellae, with flattened,
branched, connective tissue corpuscles, clasping the
bundles of fibrils. Both its inner and outer surfaces
are smooth and covered by epithelial plates. Many
lymphatics and blood-vessels, as well as slender
nerve filaments derived from both spinal and sym-
pathetic systems, are furnished to its substance.

II.— THE PIA MATER.

(Figs. 2 to 6.)

Disskotion, — After slitting open the dura mater, the first mem-
brane you will see is the delicate arachnoid. Pierce it with a
sharp-pointed blow-pipe and inflate, as far as you can, the sub-
arachnoid space. Next, to expose the pia mater, remove a small
piece of the arachnoid from any part of the cord, leaving it intact
elsewhere.

The Pia Mater is a delicate, highly - vascular,
fibrous membrane, which so closely surrounds the
spinal cord that it cannot easily be stripped off. It
also furnishes sheaths to the roots of the spinal
nerves. A distinct process or fold of it passes
into the anterior median fissure of the cord, and
other smaller septa penetrate the spinal marrow
at various points, carrying with them blood-vessels
and lymphatics for the nutrition of both its white
and grey matter. The largest of these septa is seen
at the posterior median fissure (Fig. 6, Plate II.,
page 8). It is not, however, a fold of pia mater
like that in the anterior fissure, but consists of the



AND ITS MEMBRANES. 7

supporting tissue of the cord called neuroglia (see
page 30). If you compare the pia mater of the
cord with that of the brain, you will find that the
former is thicker, less vascular and more adherent
to the subjacent nervous tissue.

The outer surface of the pia mater is compara-
tively rough, and has the three following structures
connected with it — the linea splendens, the liga-
mentum denticulatum, and the septum posticum.

1. The linea splendens (Fig. 3, page 4) is a
thickened fibrous band along its anterior aspect.
It is sometimes difficult to make out.

2. The ligamentum denticulatum is a well-
defined structure (Figs. 3 and 5, lig. dent.) which
runs longitudinally on each side of the cord in the
form of a toothed white band, having its serrated
edge turned outwards. It helps to support the
spinal marrow within its dural sheath. Internally,
it is attached to the pia mater, about midway
between the lines of origin of the anterior and
posterior nerve roots, reaching upwards as high as
the medulla oblongata, and ending below on the
pointed extremity of the cord, the conus medullaris.
Externally, its outer margin forms a series of tooth-
like processes, about twenty-one in number, which
are fixed to the inner surface of the dura mater, in
the intervals between the points of exit of successive
nerve roots. The highest of these denticulations is
attached opposite the margin of the foramen
magnum, between the last cranial and first spinal
nerve, and the lowest between the twelfth dorsal and
first lumbar nerves. The ligamentum denticulatum



8 THE SPINAL CORD

thus partially divides the sub-arachnoid space into an
anterior and a posterior compartment.

3. At the back of the cord is another process or
partition — the sejrtum posticum (Figs. 4, 6, page 8,
sep. post.) — which crosses the sub-arachnoid space
and serves to connect the pia mater with the
arachnoid.

Below the end of the cord, the pia mater, though
at first retaining its tubular form, afterwards becomes
suddenly reduced in size, and is finally prolonged as
a sheath to the delicate thread-like continuation of
the spinal cord, the filum terminate or central
ligament (Fig. 1) ; the silvery hue of which will
enable you to distinguish it among the surrounding
bundles of nerve roots (cauda equina).

Lying between the pia mater on the one hand,
and the arachnoid on the other, but connected with
both, is a quantity of delicate connective tissue
arranged in the form of a spongy network, the
sub - arachnoid trabecular, which are clothed with
epithelial plates (Fig. 4, page 8, sub-arach. trab.).
The lacuna? or areola?, thus formed, contain the
greater part of the cerebro-spinal fluid.

Minute structure of the pia mater (Fig. 27,
Plate VI., page 32). — The pia mater consists of a
basis of white fibrous connective tissue, which is
arranged in interlacing bundles, and supports a
plexus of blood-vessels. Both its surfaces are
covered by epithelial cells. It possesses a com-
plete network of lymphatics ; and its nervous
supply is derived from the sympathetic system.



Fig. 4.



"Epithelium
iganientu



racknoid train ■



Dura mater.
Pia mater.



nal nerve.




nation.



Fig. 5.



Plate II.




I) lira



gamentwtn
dent

nerve.



Fig. 6.



Arachnoid.



interior median fissure.
Dura mater.
Pia mater.




Ligament .
• dent.



Fig. 7.




ibarachnoid space.

Subdural space.



Transverse section of lower part
of spinal canal.



Fig. 8.
Interior median fiss. Anterior nerve root.



Septum posticum.
Posterior median fissure.




rang/ion. ;

Spina! nerve.
Posterior median fins. Posterior nerve root.



AND ITS MEMBRANES. 9

III.— THE ARACHNOID.

(Figs. 2 to 6.)

The Arachnoid is an extremely fine and delicate
membrane. It is non-vascular, and thus differs
materially from the other two. Many authors deny
that it is a special membrane, and consider it to be
one of the layers of the pia mater, describing the
two together as the pia-arachnoid (see pages 50
and 51).

Forming a cylindrical partition between the dura
mater and the pia mater, the arachnoid divides the
space between them into two — the sub-dural and
sub-arachnoid, previously referred to. The sub-dural
space is very narrow, for the outer surface of the
arachnoid is in more or less close contact with the dura
mater. The sub-arachnoid space is much larger and
contains the chief part of the cerebro-spinal fluid.
It is crossed by the sub-arachnoid trabecular which
connect the arachnoid with the pia mater. The arach-
noid, moreover, forms tubular prolongations around
both the nluni terminale and the teeth-like processes
of the ligamentum denticulatum ; and similar cover-
ings are furnished to the roots of the spinal nerves.
These sheaths enclose the nerve roots as they pass
outwards to the dura mater, but, when the roots
pierce that membrane, the epithelium of the outer
surface of the arachnoid becomes continuous with
the epithelium lining the inner surface of the dura
mater, whilst the rest of the arachnoidal sheath
blends with the perineurium of the nerves (Figs. 2
and 4. epith.). Thus we see that each spinal nerve



io THE SPINAL CORD

root receives a covering from all tbree membranes
of the cord. It is stated, too, that the sub-dural and
sub-arachnoid spaces, though they do not directly
communicate with one another, are both continuous
with the lymphatic plexuses which surround the
spinal nerves. It may perhaps be well to mention
that the sub-dural space was formerly spoken of as
"the cavity of the arachnoid" — the arachnoid
membrane itself being then looked upon as a
serous membrane, enclosing a serous cavity, and
the epithelial lining of tbe deep surface of the
dura mater as one of its layers.

Minute structure of the arachnoid. — The
arachnoid consists of bundles of white fibrous tissue,
interlacing with one another, and arranged for the
most part longitudinally. Both its surfaces are
covered by epithelial cells. The source from which
it derives its nerve-supply is still very doubtful;
most probably it is the sympatbetic system. As
we have said, many authors regard the arachnoid
as one of the layers of the pia mater.

Cerebro-spinal Fluid. — The cerebro-spinal fluid,
about two fluid ounces in quantity, is a clear-looking,
alkaline liquid, containing little or no albumen. It
chiefly occupies the interstices of the sub-arachnoid


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