424 DISEASES OF THE NERVOUS SYSTEM. 



cortex of the brain is congested and the convolutions flattened 

 by pressure from within. 



At times large caseous nodules are found in the pia mater 

 and brain-tissue. 



Syphilitic inflammation of the meninges is generally circum- 

 scribed. The gummata which form involve the pia and 

 the cerebral cortex, or they may extend outward and involve 

 the dura. 



Tumors : Endothelioma is probably the tumor most fre- 

 quently met in the meninges. Lipomata, fibromata, and 

 myxomata are of rare occurrence. 



THE BRAIN. 



THROMBOSIS AND EMBOLISM. 



The results of thrombosis and embolism in the brain may 

 be most serious, producing death, or more or less extensive 

 paralyses. 



Emboli are brought most frequently from fibrinous vegeta- 

 tion on the cardiac valves, or from cardiac or aneurysmal 

 thrombi. They generally lodge in the left middle cerebral 

 artery. 



Thrombosis is most frequent in the basilar artery, but may 

 occur anywhere, as the result of the presence of an embolus, 

 or some local inflammatory or degenerative alteration of the 

 vessel-wall at the site of its formation. 



In either case the result of this obstruction of the blood- 

 supply is a rapid necrotic softening of the area affected 

 encephalomalacia. The nerve-cells degenerate and lose their 

 axis-cylinder processes; the myelin-sheaths of the nerve- 

 fibres undergo fatty degeneration ; and later the neuroglia- 

 fibres a similar change. A more or less pigmented scar is 

 finally formed, composed mainly of neuroglia-tissue. In the 

 case of an infectious embolus, an abscess may form. These 

 areas of softening are usually red from the extravasation of 

 blood, as in infarcts in other organs ; at a later stage when 

 most of the pigment has been absorbed, or when little or no 

 blood has been extravasated, they are yellow or white. 



