THE BRAIN AND ITS MENINGES 223 



THE BRAIN AND ITS MENINGES. 



The Dura Mater is a fibro-serous membrane. The fibrous 

 layer forms the endo-periosteum of the skull, to which it is 

 attached by numerous fibrous processes, but it is only strongly 

 adherent over the floor of the cranial fossae and along the suture 

 lines. The meningeal vessels lie between the dura mater and 

 the bone, and, owing to the ease with which the membrane 

 may be stripped off, extra-dural blood-clots may attain a suffi- 

 ciently large size to exert a fatal degree of intra-cranial pressure. 

 The serous layer forms a covering for the brain and supports the 

 thin endothelial walls of the cranial blood sinuses, which, conse- 

 quently, do not collapse when they are wounded. The falx 

 cerebri is a fold of the serous layer, which passes downwards in 

 the middle line between the two cerebral hemispheres. Pos- 

 teriorly, a similar fold, termed the tentorium cerebelli, projects 

 inwards between the cerebellum and the cerebrum ; and the 

 two cerebellar hemispheres are partially separated by the falx 

 cerebelli, a small fold which extends from the internal occipital 

 protuberance to the posterior border of the foramen magnum. 



The Superior Sagittal (Longitudinal) Sinus lies in the 

 upper margin of the falx cerebri (Fig. 68). It begins at the fora- 

 men caecum, where it communicates with the nasal veins (p. 

 221), and runs backwards to the internal occipital protuberance. 

 At this point it communicates with the straight sinus, and turns 

 laterally, usually to the right, to form the transverse sinus. 

 It is half an inch in breadth posteriorly, and, therefore, trephine 

 openings should not be made less than three-quarters of an inch 

 from the median line. In addition to the parietal emissary vein 

 (p. 220), it receives numerous cerebral veins, which run forwards 

 and medially to enter the sinus. Backward movements of the 

 head, which tend to empty the sinus, retard the outflow from 

 the cerebral veins, owing to their forward inclination. 



The Inferior Sagittal Sinus lies in the lower border of the 

 falx cerebri, and is joined posteriorly by the great cerebral 

 vein (of Galen) to form the Straight Sinus, which runs back- 

 wards on the upper surface of the tentorium cerebelli and in the 

 base of the falx cerebri. At the internal occipital protuberance 

 the straight sinus bends laterally to form the transverse sinus, 

 usually of the left side, but at its termination it communicates 

 freely with the superior sagittal sinus. 



