THE NERVOUS SYSTEM. 



795 



Iii general, the first effect of the occlusion of an artery is to deprive 

 the region to which it is distributed of blood. In arteries whose branches 

 anastomose, the affected area is soon supplied with blood by the estab- 

 lishment of a collateral circulation. In terminal arteries, on the other 

 hand, the blocking of the vessel is followed, as a rule, by degenerative 

 changes and softening in the brain tissue. The appearances which these 

 degenerated areas present vary greatly, depending upon the stage of the 

 degeneration and the amount of blood which may be extra vasated. Dense 



FIG. 521. DEGENERATED CELLS, CHOLESTERIN CRYSTALS, AND CORPORA AMYLACEA FROM BRAIN TIS- 

 SUE IN EMBOLIC SOFTENING. 



a. Fatty ganglion cells ; 7>, corpora amylacea ; c, cell containing very large number of fat droplets (com- 

 pound granular or Gluge's corpuscles) ; d, cholesterin crystals. 



infiltrations of the brain tissue with blood, as in ha3morrhagic infarctions 

 from emboli in other parts of the body, do not usually occur, although 

 considerable blood may be extravasated. Areas of softening in which 

 there is little extravasation of blood are usually white or yellow in color, 

 white or yellow softening. "When much blood is present the process is 

 frequently called red softening. Yellow softening is probably often only 

 a secondary stage of the red, resulting from absorption of most of the 

 pigment. The term "white softening" is often used interchangeably 

 with "yellow." It is also applied to that condition into which the latter 

 passes after more complete liquefaction, and after fatty changes have 

 taken place. Young proliferating neuroglia tissue often gives a glisten- 

 ing white appearance to the lesion. 



The tissue in the affected area gradually softens and may become dif- 

 fluent. Microscopically, the softened tissue is seen to consist of more or 

 less fluid with broken-down brain tissue, fragments of nerve fibres, drop- 



