DUKA MATER. 669 



sagittal sinus ; whilst along its attachment to the tentorium is enclosed the straight 

 sinus. 



The tentorium cerebelli is a large crescentic partition of dura mater, which forms 

 a membranous tent-like roof for the posterior cranial fossa, and thus intervenes 

 between the posterior portions of the cerebral hemispheres and the cerebellum. It 

 is accurately applied to the superior surface of the cerebellum. Thus, its highest point 

 is in front and in the median plane, and from this it slopes downwards towards its 

 attached border. It is kept at a high degree of tension, and this depends on the 

 integrity of the falx cerebri, which is attached to its superior aspect in the median 

 plane. 



The posterior border of the tentorium is convex, and is attached to the hori- 

 zontal ridge which marks the deep surface of the occipital bone. Beyond this, on 

 each side, it is fixed to the postero-inferior angle of the parietal bone, and then 

 forwards and medially 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 sinus transversus, whilst along 

 the superior border of the petrous bone it encloses the superior petrosal sinus. The 

 anterior border of the tentorium is sharp, free, and concave, and forms with the 

 dorsum sellse an oval opening shaped posteriorly like a pointed arch. This opening 

 receives the name of the incisura 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 medially, 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 median plane from the internal occi- 

 pital crest. It occupies the notch which separates the two hemispheres of the 

 cerebellum posteriorly. Inferiorly it bifurcates into two small diverging ridges 

 which gradually fade away as they are traced forwards on each side of the foramen 

 magnum. 



The diaphragma sellse is a small circular fold of dura mater which forms a roof 

 for the fossa hypophyseos. A small opening is left in its centre for the trans- 

 mission of the infundibulum. 



Dura Mater Spinalis. In the vertebral canal the dura mater forms a tube which 

 encloses the spinal medulla, 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 

 applied to the spinal medulla 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 thoracic 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 superior end of the vertebral canal the spinal 

 dura mater is firmly fixed to the third cervical vertebra, to the epistropheus 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 inferior 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 vertebral canal its 

 two constituent layers separate. The inner layer is carried downwards as the long 

 cylindrical tube which encloses the spinal medulla. 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 ligaments 

 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 vertebral canal by an interval, the cavum epidurale, which is occupied by 



