THE DURA MATER 109 



contain a clear serous fluid (p. 23), which serves to protect 

 the brain from laceration and contusion. 



The Dura Mater constitutes the outermost covering and is 

 the strongest of the three membranes which invest the brain. 

 It is usually described as consisting of an inner ^ serous and 

 an outer, fibrous layer, but the latter is really the endo- 

 periosteum, which lines the cranial cavity and is everywhere 

 closely applied to the bone, being specially adherent to the 

 floor of the skull. 



The serous layer of the dura mater lines the cavity in which 

 the brain lies, and it is separated from the arachnoid by the 

 subdural space, which contains the clear, subdural fluid. 

 No communications exist between the subdural and the sub- 

 arachnoid spaces, and the passage of fluid from one space 

 to the other occurs by a process of osmosis through the 

 arachnoid. There is no marked difference between the sub- 

 dural fluid and the cerebro-spinal fluid, which is found in the 

 subarachnoid space, and it is therefore immaterial which of 

 the two is drawn off for examination in a lumbar puncture 

 (p. 41). In fractures of the skull, the discharge of serous 

 fluid from the nose or acoustic meatus indicates that the serous 

 layer of the dura mater has been injured. 



The falx cerebri is a longitudinal crescentic fold of the 

 serous layer. It dips into the longitudinal fissure and par- 

 tially separates the two cerebral hemispheres from one another. 

 Its anterior extremity is attached to the ethmoid bone, but its 

 intermediate portion has a free lower margin, which overhangs 

 the corpus callosum. Posteriorly, the two layers of the falx 

 cerebri are continuous, on each side, with the upper layer of 

 the tentorium cerebelli (Fig. 53). 



The tentorium cerebelli is a transverse fold of the serous 

 layer, which projects into the posterior part of the cranial 

 cavity from behind and from the sides. It forms a partition 

 which separates the cerebellum below from the cerebral hemi- 

 spheres above. Its peripheral border is attached to the 

 upper margin of the posterior cranial fossa, but its anterior 



