558 THE NERVOUS SYSTEM. an | 
zontal ridge which marks the deep surface of the occipital bone. Beyond this, on 
each side, it is fixed to the postero-inferior border of the parietal bone, and then 
forwards along the superior border of the petrous portion of the temporal bone. 
From the internal occipital protuberance to the postero-inferior angle of the 
parietal bone this border encloses the /ateral blood sinus, whilst along the upper 
border of the petrous bone it encloses the superior pelrosal sinus. The anterior 
border of the tentorium is sharp, free, and concave, and forms with the dorsum 
sellze an oval opening shaped posteriorly like a pointed arch. This opening receives 
the name of the ineisura tentorii, and within it is placed the mesencephalon, or the 
stalk of connexion between the parts which lie in the posterior cranial fossa and 
the cerebrum. Beyond the apex of the petrous part of the temporal bone the two 
margins of the tentorium cross each other like the limbs of the letter x; the free 
margin is continued forwards, to be attached to the anterior clinoid process, whilst 
the attached border proceeds inwards, to be fixed to the posterior clinoid process. 
The falx cerebelli is a small, sickle-shaped process of dura mater placed below 
the tentorium, which projects forwards in the mesial plane from the internal occi- 
pital crest. It occupies the notch which separates the two hemispheres of the 
cerebellum posteriorly. Inferiorly it bifureates into two small diverging: ridges 
which gradually fade away as they are traced forwards on either side of the 
foramen magnum. 
The diaphragma sellz is a small circular fold of dura mater which forms a roof 
for the pituitary fossa. A small opening is left in its centre for the transmission of 
the infundibulum. 
Spinal Dura Mater (dura mater spinalis)—In the spinal canal the dura mater 
forms a tube which encloses the spinal cord, and which extends from the foramen 
magnum above to the level of the second or third piece of the sacrum below. It 
is very loosely appled to the spinal cord and the nerve-roots which form the 
cauda equina; in other words, it is very capacious in comparison with the volume 
of its contents. Moreover, its calibre is not uniform. In the cervical and lumbar 
regions it is considerably wider than in the dorsal region, whilst in the sacral 
canal it rapidly contracts, and finally ends by blending with the filum terminale 
externum, the chief bulk of which it forms. At the upper end of the spinal 
canal the spinal dura mater is firmly fixed to the third cervical vertebra, to the 
axis vertebra, and around the margin of the foramen magnum. In the sacral 
canal the filum terminale externum, with which it blends, extends downwards to 
the back of the coccyx, to the periosteum of which it is fixed. The lower end of 
the tube is thus securely anchored and held in its place. 
Within the cranial cavity the dura mater is closely adherent to the bones, and forms for them 
an internal periosteum. As it is followed into the spinal canal its two constituent layers 
separate. The inner layer is carried downwards as the long cylindrical tube which encloses the 
spinal cord. The outer layer, which is much thinner, becomes continuous behind and on each 
side of the foramen magnum with the periosteum on the exterior of the cranium, whilst in front 
it is prolonged downwards into the vertebral canal in connexion with the periosteum and liga- 
ments on the anterior wall of the canal. The spinal dura mater, therefore, corresponds to the 
inner layer of the cranial dura mater, and to it alone. It is separated from the walls of the 
spinal canal by an interval, the epidural space, which is occupied by soft fat and a plexus of thin- 
walled veins. In connexion with the spinal dura mater there are no blood-sinuses such as are 
present in the cranial cavity, but it should be noted that the veins in the epidural space, placed 
as they are between the periosteum of the spinal canal and tube of dura mater, occupy the same 
morphological plane as the cranial blood-sinuses. Another feature which serves to distinguish 
the spinal dura mater from the cranial dura mater consists in the fact that it gives off from its 
deep surface no partitions or septa. 
The cylindrical tube of spinal dura mater does not lie absolutely free within 
the vertebral canal. Its attachments, however, are of such a character that they 
in no way interfere with the free movement of the vertebral column. On either 
side the spinal nerve-roots, as they pierce the dura mater, carry with them into the 
intervertebral foramina tubular sheaths of the membrane, whilst in front loose fibrous 
prolongations—more numerous above and below than in the dorsal region—connect 
the tube of dura mater to the posterior common ligament of the vertebral column. 
No connexion of any kind exists between the dura mater and the posterior wall of 
the spinal canal. 
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