DUEA MATER. FALX.-TEXTOKIUM. 563 



arachnoid, and with it, therefore, forms a fibro-serous membrane, which is 

 free, smooth, and epitheliated on its inner surface, where it is turned 

 towards the brain and cord, but which, by its outer surface, is connected 

 with the surrounding parts, in a somewhat different manner in the cranium 

 and in the spinal canal. 



The outer surface of the cranial portion adheres to the inner surface of 

 the bones, and forms their internal periosteum. The connection between 

 the two depends, in a great measure, on blood-vessels and small fibrous 

 processes, which pass from one to the other ; and the dura mater, when 

 detached and allowed to float in water, presents a flocculent appearance on 

 its outer surface, iu consequence of the torn parts projecting from it. The 

 adhesion between the membrane and the bone is more intimate opposite 

 the sutures, and also generally at the base of the skull, which is uneven, 

 and perforated by numerous foramina, through which the dura mater is 

 prolonged to the outer surface, being there continuous with the pericranium. 

 The fibrous tissue of the dura mater becomes blended with the areolar 

 sheaths of the nerves, at the foramina which give issue to them. 



In leaving the skull, the dura mater is intimately attached to the margin 

 of the foramen magnum ; but within the vertebral canal it forms a loose 

 sheath around the cord (#ieca), 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 vertebrae. The space intervening between the wall of the 

 canal and the dura mater is occupied by loose fat, by watery areolar tissue, 

 and by a plexus of spinal veins. 



Opposite each iutervertebral foramen the dura-matral theca presents two 

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

 correspond! Lg spinal nerve. It is continued as a tubular prolongation on, 

 the nerve, 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 divi- 

 sible into two distinct layers, and at various places these layers separate 

 from each other and leave intervening channels, called sinuses. These 

 sinuses, which have been elsewhere described, are canals for venous blood, 

 and are lined with a continuation of the internal membrane of the veins. 



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

 strong membranous processes, or partitions^ formed by duplication of its 

 inner layer. 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 thj falx cerebri. The second is an arched or vaulted 

 partition, stretched across the back part of the skull, between the cerebrum 

 and the cerebellum ; it is n amed the tentorium cerebelli. Below this, another 

 vertical partition, named falx cerebelli, of small extent, passes down between 

 the hemispheres of the cerebellum. 



The /ate cerebri 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 

 situated. Along its upper convex border, which is attached above to the 

 middle line of the inner surface of the cranium, runs the supeiior longi- 

 tudinal sinus. Its under edge is free, and reaches to within a short distance 

 of the corpus callosum, approaching nearer to it behind. This border con- 

 tains the inferior longitudinal sinus. 



The tentorium, or tent, is elevated in the middle, and declines downwards 



