182 THE CEREBRAL HEMISPHERES. 



the foramen magnum, and below this to the cervical vertebras as far as the third. 

 Above the atlas it has an orifice on each side for the passage of the vertebral artery. 

 Within the rest of the vertebral canal it forms a loose sheath around the cord (theca), 

 and is not adherent to the bones, which haVe an independent periosteum. Towards 

 the lower end of the canal, a few fibrous slips proceed from the outer surface of the 

 dura mater to be fixed to the vertebras ; one such being especially well marked at the 

 lower end, and seeming to join the anterior surface of the dura mater to the posterior 

 common ligament of the vertebrae (anterior ligament of the dura mater, Trolard). 

 The theca ends opposite the second sacral vertebra in the adult (see p. 6). The 

 space intervening between the wall of the canal and the dura mater (epidural space) 

 is occupied by loose fat, by areolar tissue, and by a plexus of spinal veins. 



Opposite each intervertebral foramen the dura-matral theca has two openings, 

 placed side by side, which give passage to the two roots of the corresponding spinal 

 nerve. It is continued as a tubular prolongation on each nerve (fig. 128), and is 

 lost in its sheath. Besides this, it is connected with the circumference of the 

 foramen by areolar tissue. 



The fibrous tissue of the dura mater, especially within the skull, is divisible into 

 two distinct layers, and at various places the layers separate from each other and 

 leave intervening channels, called sinuses. These sinuses, which have been else- 

 where described (Vol. II.), are channels for venous blood, and are lined with a 

 continuation of the endothelium of the veins. The division into two layers is most 

 complete at the base of the skull, in the middle fossa, and in the neighbourhood of 

 the cavernous sinus ; on the outer side of this the Gasserian ganglion is included in 

 a space (cavum Meckdii) between the two layers. Between the two cavernous 

 sinuses the pituitary body is received into a depression of the membrane, which 

 closely surrounds the organ in question, except where the infundibulum enters it. 

 There is further a fissure immediately over the orifice of the aquasductus vestibuli, 

 and here the prolongation of the membranous labyrinth of the ear, known as the 

 saccus endolymphaticus, is received between the two layers. 



The dura mater also sends inwards into the cavity of the skull three strong 

 membranous processes or partitions. Of these, one descends vertically in the median 

 plane, and is received into the longitudinal fissure between the two hemispheres of 

 the cerebrum. This is the falx cerebri. The second is a sloping vaulted partition, 

 stretched across the back part of the skull, between the cerebrum and the cerebellum, 

 named the tentorium cerebelli. Below this, another vertical partition, named falx 

 cerebelli, of small extent, passes down between the hemispheres of the cerebellum. 

 Lastly, the portion of dura mater which stretches over the sella turcica, and pierced 

 by a small hole for the infundibulum, covers the pituitary body, is sometimes spoken 

 of as the operculum or tentorium of the hypophysis. 



The falx cerebri (fig. 129, 1) is narrow in front, where it is fixed to the crista 

 galli, and broader behind, where it is attached to the middle of the upper surface of 

 the tentorium, along which line of attachment the straight sinus is attached. 

 Along its upper convex border, which is attached to the middle line of the inner 

 surface of the cranium, runs the superior longitudinal sinus. Its under edge is 

 free, and reaches to within a short distance of the corpus callosum, approaching 

 nearer to it behind. This border contains the inferior longitudinal sinus. 



The tentorium cerebelli, or tent (fig. 129, 8), is elevated in the middle, and 

 declines downwards in all directions towards its circumference, thus following the 

 form of the upper surface of the cerebellum. Its inner border is free and concave, 

 and leaves in front of it a shield-shaped opening, through which the isthmus encephali 

 extends. It is attached behind and at the sides by its convex border to the hori- 

 zontal part of the crucial ridges of the occipital bone, and there encloses the lateral 

 sinuses. Farther forward it is connected with the upper edge of the petrous portion 



