THE NEBVOUS SYSTEM. 771 



the ventricles persist for days and weeks after the subsidence of the in- 

 flammation of the pia mater. 



For a discussion of the bacterial excitants of exudative meningitis 

 see page 218. ' 



Chronic Meningitis. Either the pia mater at the base of the brain 

 alone may be inflamed (basilar meningitis), or the pia mater over the 

 convexity alone, or the entire pia mater, or circumscribed patches of the 

 membrane. The pia mater is thickened and opaque, the thickening 

 being sometimes very considerable. There is a formation of new connec- 

 tive tissue and this may be associated with accumulation of pus, fibrin, 

 and serum ; the relative quantity of these inflammatory products varies 

 in different cases. Firm and sometimes extensive adhesions may be 

 formed between the dura mater and the pia mater. Not infrequently the 

 cortical portions of the brain participate in the process, and there is an 



FIG. 506. FATTY DEGENERATION OF CELLS ALONG THE BLOOD-VESSELS OF THE PIA MATER AFTER 

 EXUDATIVE MENINGITIS. 



From the pia mater of a child five years old. 



infiltration of small spheroidal cells around the blood-vessels, thickening 

 of the walls of the vessels, and degenerative changes and atrophy of the 

 nerve tissue. New connective tissue may also form in the brain sub- 

 stance, which may become closely adherent to the pia mater. The ven- 

 tricles of the brain also may contain an increased amount of serum and 

 may be dilated ; the ependyma may be thickened and roughened. This 

 form of inflammation may be the result of injury or disease of the cra- 

 nial bones, or secondary to chronic pachymeningitis or to inflammation of 

 the brain substance. It may occur in the vicinity of tumors of the brain 

 or meninges. It may be a complication of chronic diffuse nephritis or 

 the result of chronic alcoholic poisoning. It may occur in marked form 

 in general paresis of the insane. 



1 The close topographic relationships which the nasal cavities and the middle ear 

 bear to the meninges are significant in this connection on account of the possibility of 

 the transmission to the brain membranes of bacteria not uncommonly present and usu- 

 ally harmless in the former situations. 



