THE NERVOUS SYSTEM. 



763 



may occur in the infectious and exhausting diseases. It may occur in 

 apparently healthy persons without discoverable cause. The thrombi 

 may be red or white and firm. They may induce no secondary changes, 

 or they may extend into the veins and induce hseniorrhagic softening of 

 the involved areas of the brain. 



INFLAMMATION. (Pachymeningitis.) 



This may involve the external layers of the membrane, pachymenin- 

 gitis externa, or the internal layers, pacliymeningitis interna. It may be 

 either acute or chronic. The tissues of the substance of the dura mater 

 participate to a greater or less degree in these changes, but the chief 

 lesions are upon the surfaces. 



Acute pachymeningitis externa is usually secondary to injuries or dis- 

 eases of the cranial bones ; thus it may be incited by fractures of the 



FIG. 500. PACHYMENINGITIS INTERNA HJCMORRHAGICA CHRONIC. 

 At the left near the vessels are connective-tissue cells containing blood pigment. 



skull, either depressed or not, osteitis, caries, suppurative inflammation 

 of the internal and middle ear and of the mastoid cells. The dura mater 

 is commonly congested and swollen, and may contain small ecchymoses. 

 The inflammation is usually suppurative, and pus may accumulate be- 

 tween the membrane and the bone, or in the substance of the membrane. 

 The areas of inflammation are not usually extensive. It sometimes 

 leads to thrombosis of the venous sinuses, and sometimes gangrene of the 

 dura mater occurs. The inflammation may extend to the inner surface 

 of the dura mater, to the pia mater and brain, or it may remain localized 

 and undergo resolution. 



Acute pachymeningitis interna may be secondary to inflammation of the 

 external surface, or it may occur as a complication in pyaemia, puerperal 

 fever, chronic diffuse nephritis, in the exanthemata and erysipelas, or 

 independently. There is a general or circumscribed production of fibrin 



