764 THE NERVOUS SYSTEM. 



and pus, so that the internal surface of the membrane is lined with a 

 layer of soft, yellow exudate. 



Simple chronic pachymeningitis consists in the formation of new con- 

 nective tissue in the dura mater, by which it becomes thicker and in 

 many cases abnormally adherent to the bones of the skull. This thick- 

 ening may be general or circumscribed, and may involve the entire 

 thickness of the membrane. Not infrequently, when the external layers 

 are especially involved, firm adhesions to the skull occur, with ossifica- 

 tion of the outer layers, so that shreds of the membrane containing little 

 masses of bone (osteophytes) remain sticking to the skull when the mem- 

 brane is stripped off. 



Pachymeningitis Interna Haemorrhagica, This is an important form 

 of chronic inflammation of the internal layer of the dura mater, charac- 

 terized by the formation of layers of new deli- 

 cate connective tissue with numerous very thin- 

 walled blood-vessels from which the blood is 

 prone to escape. The membrane may at first 

 appear as a delicate fibrinous pellicle, with small 

 red spots scattered through it, or it may look 

 like a simple reddish or brown staining of the 

 inner surface of the dura mater. Microscopical Fir ^ _ BRAIX SASD FROM 

 examination shows this membrane to consist of PACHYMEXIXGITIS IXTERNA. 

 numerous blood-vessels, mostly capillaries with 



very thin walls, which may be distended or pouched, and which have 

 grown out from the vessels of the dura mater (Fig. 500). Between 

 the vessels is a homogeneous or slightly differentiated basement sub- 

 stance, containing a variable number of spheroidal, fusiform, or branch- 

 ing cells. Bed blood cells in variable quantity, blood pigment in 

 various forms, frequently enclosed in the new cells, and small calcareous 

 concretions (brain sand) (Pig. 501), also lie in the intervascular spaces. 

 In more advanced stages the new membrane may become greatly thick- 

 ened, its outermost layers being changed into dense fibrous tissue with 

 obliteration of the vessels; while the more recently formed layers are 

 similar in structure to those at first developed. Considerable blood usu- 

 ally escapes by diapedesis from the vessels of the new membrane in all 

 stages of its formation. The vessels are also very liable to rupture, 

 giving rise to extensive hemorrhages either into the substance of the 

 membrane or between it and the pia mater. Sometimes masses of new 

 tissue and blood, from half an inch to an inch or more in thickness, are 

 in this way formed, greatly compressing the brain. These new mem- 

 branes are most frequently formed over the convexity of the brain, but 

 may extend over nearly the entire surface of the dura mater. Some- 

 times, when old, the entire membrane, densely pigmented and firm, lies 

 loosely beneath the dura mater without compressing the brain or giving 

 any clinical indication of its presence. The membrane may induce 

 chronic changes in the pia mater, with or without accompanying changes 

 in the cortical portion of the brain. 



