770 



THE NERVOUS SYSTEM. 



a sort of cast of the brain surface at the seat of the lesion (Fig. 505). 

 Sometimes extravasated red blood cells are mingled with the other exu- 

 dations, as the result of diapedesis. Microscopical examination shows 

 numerous white blood cells sticking in the walls of the veins and capil- 

 laries, or the vessels may be blocked with them. It is evident that a 

 large part of the pus cells accumulate as the result of emigration. The 

 connective -tissue cells of the pia mater may be detached from their 

 places or degenerated. In some cases there are considerable accumula- 

 tions of pus between the pia mater and the brain substance and along 

 the vessels which enter the latter. More rarely pus is found upon the 

 free surface of the membrane. The brain substance may be compressed 

 by the accumulated exudate, so that the convolutions are flattened. 

 The cortical portion of the brain may be simply infiltrated with serum 





WiO, 505. ACUTE EXUDATIVE MENINGITIS. 



o. Convolutions of cerebrum ; fr, pia mater thickly infiltrated with pus ; c, blood-vessels entering brain 

 from pia and surrounded by a zone of pus cells ; d, congested blood-vessels of pia mater ; e, smaller blood- 

 vessels of pia, around which pus cells are collected in dense masses. 



cedematous or it may undergo degenerative changes, or it may be 

 the seat of punctate haemorrhages. Not infrequently the inflammation 

 extends to the ventricles, which may contain purulent seruin, and to the 

 pia mater of the cord. This form of inflammation is most frequent on 

 the convexity of the brain, but may extend or even be confined to the 

 base. It may be localized, but frequently extends widely over the -sur- 

 faces of the hemispheres. 



When recovery from acute exudative meningitis occurs there may be 

 fatty degeneration of the cells which have accumulated in the pia mater, 

 particularly along the vessels (Fig. 506), and this may produce white 

 patches in the membrane and threads along the blood-vessels, which 

 resemble the appearance of an accumulation of exudate in the acute 

 stage. Fatty degeneration of the blood-vessels and cells of the pia 

 mater may also occur without acute inflammatory changes. 



Sometimes, in children and young adults, inflammatory changes in 



