628 
The spinal dura mater is in shape adapted 
to the vertebral canal. It is a hollow cylinder, 
tapering somewhat at its lower extremity to cor- 
respond with the sacral portion of the canal. 
Itadheres very firmly all round the foramen mag- 
num of the occipital bone. From thence it 1s 
continued down to the sacrum without forming 
any adhesion to bone. On the posterior and 
lateral surfaces it is covered by a layer of soft, 
oily, reddish fat, which intervenes between it 
and the inner surfaces of the vertebral lamin 
and processes, and in these situations, as well 
as to a less degree in front, we find a very in- 
tricate plexus of veins, some of which are of 
considerable size. The fatty deposit is most 
abundant in the sacral region. In front the dura 
mater adheres by a close areolar tissue to the 
posterior common ligament, and here of course 
the adipose tissue is deficient. At the foramen 
magnum the continuity of the spinal dura 
mater with that of the cranium is distinct, and 
here, indeed, the former appears as a funnel- 
shaped prolongation of the latter; both are, 
in trath, portions of the same membrane 
adapted to the difference of shape of the ner- 
vous centres with which they are respectively 
connected. 
On the sides the spinal dura mater is per- 
forated by orifices which give exit to the roots 
of the nerves which arise from the spinal cord. 
When examined from within, these foramina 
are found to be arranged in pairs, each pair 
corresponding to the point of exit of a spinal 
nerve. The foramen which transmits the an- 
terior root is separated from that which gives 
passage to the posterior one, by a narrow slip 
of fibrous membrane. These foramina are 
slit-like in form, taking the vertical direction. 
On the outer surface of the dura mater the 
distinction between them is not evident with- 
out dissection, for there the fibrous membrane 
being prolonged from the margins of the open- 
ings around the nerves, the sheaths thus formed 
coalesce and surround both roots. The number 
of these orifices is of course the same as that 
of the roots of the nerves which pass through 
the dura mater. 
The internal surface of the spinal dura mater 
is perfectly smooth and moist in the healthy 
state, Owing to its being lined by the parietal 
layer of the arachnoid membrane. In the 
intervals between the orifices for the transmis- 
sion of each pair of spinal nerves, it receives the 
pointed attachments of the ligamentum denti- 
em to be described more fully by-and- 
e. 
rhe is evident from the preceding description 
that the spinal dura mater cannot perform the 
office of a periosteum to the osseous walls of the 
spinal canal, for at every point it is separated 
from them by texture of a different kind, and, 
moreover, the vertebra are provided with a 
distinct periosteum. The prolongations of dura 
mater over the nerves at each of the interverte- 
bral foramina serve to fix that membrane at 
the sides throughout the whole extent of the 
vertebral canal, so as to prevent its lateral dis- 
placement. At the lower extremity of the 
sacral canal the dura mater ends in a blunt 
NERVOUS SYSTEM. (Nervous Centres. Tur Meninces.) 
oj 
4 
int, and from this certain may be 
285 towards the coecyx. Bf these the de J 
tral one is continuous with the filiform pro 5 
longation from the pia mater, which is inserted 
into the inferior extremity of the dura er, 
and is implanted oy He e last per i 
coceyx. e thread-like processes which ¢ 4 
ain 0 each side are the sheaths of the last 
sacral nerves and of the coccygeal nerve, which — 
pass some distance in the canal before pe! 
reach the foramina for their transmission 
wards. 5 
It is easy to convince oneself that the spinal — 
dura mater is far larger than would be neces- 
sary for the reception of the cord. When the 
fluid immediately surrounding this organ 
been suffered to escape, the dura mater appears” 
quite loose, flaccid, and wrinkled. By blowing — 
air or injecting water into its canal, it may be» 
rendered tense again. This looseness e 
dura mater is most conspicuous at its st 
, in the lumbar and sacral regions, where 
it forms, as Cruveilhier says, “ autour de la 
queue de cheval une vaste ampoule, qui parail 
n‘avoir d’autre utilité que de servir de 
au liquide cephalo-rachidienne.” a 
The dura mater adapts itself, in point of : 
to the varying dimensions of the s cana 
in its different regions, which Bey appear to 
be influenced by variations in dimensior 
of the spinal cord. Thus, it swells in th 
cervical and in the lumbar region, at bo 
which places there are corresponding enlarg 
ments of the cord. Its most contracted port 
is that which occupies the dorsal region. __ 
Cranial dura mater.—The dura mater of 
cranium differs in one leading cireumstal 
from that of the spine,—namely, that it fo 
a periosteum to the inner surface of the era 
bones. We find it, therefore, very clos 
adherent to the whole interior of the craniun 
and the free communication between the ves 
of the dura mater and those of the bones se 
materially to enhance the connexion bet 
this membrane and the osseous surface. 
some situations the adhesion is so very i 
that we experience great difficulty in atten 
to separate the fibrous membrane from the 
jacent bone. On the roofs of the ork 
wings of the sphenoid bone, the petr 
tions of the temporal bones, the margin 
occipital foramen, and opposite the 
the adhesion is very intimate. wll 
This adhesion of the dura mater to the 
is found also to vary in degree at differ 
tiods of life. It is very intimate in 0 
so much so that, in removing the 
layers of bone often chip off, remaining” 
nexion with the fibrous membrane. — 
adult, such a degree of adhesion as woul 
rise to this effect, ought to be regarded as 
bid. In the young subject, while i 
and growth are going on, the adhesion 
intimate, so that in them great diffic 
experienced in removing the calvaria. D 
less this intimate adhesion at this 
of life is due to the active share © 
dura mater takes in conveying the mater 
nutrition and growth to the cranial pariete 
