i8o Necrosis. 



give off a ferment into the tissues ; it is also frequently seen in 

 infarcts caused by embolism and in new growths anaemic because 

 of deprivation of their blood supply, in which necessarily it must 

 be supposed that there is produced by the tissue disintegration 

 some substance comparable to fibrin ferment. 



The saturation of a necrosed tissue by lymph is quite possible if 

 evaporation is prevented (for this reason coagulation-necrosis is most 

 commonly seen in parts enclosed from the air, internal organs and mucous 

 membranes) ; in moist, necrotic tissue the blood circulation is, it is true, 

 completely in abeyance, but the diffusion of the lymph and the immigra- 

 tion of lymph corpuscles may be expected. 



[Many, among whom the editor would include himself, dif- 

 ferentiate between coagulation-necrosis and caseation, regarding 

 the latter as essentially a further destructive change, an advanced 

 fatty degeneration of the necrotic substance. Where an area of 

 coagulation necrosis, or what is practically the same thing, hyaline 

 degeneration (at least some forms of the latter), is retained in the 

 tissue and is not further disintegrated by liquefying processes 

 it becomes the seat of f^tty degeneration and is broken down into 

 a fine detritus, oil droplets, and often contains crystals of cholesterin 

 and fatty acids. When this is of advanced degree it is described 

 imder the name of caseation. Two varieties are commonly de- 

 picted, dry and moist. Grossly a dry cheesy focus is usually de- 

 fined, often encapsulated, whitish or yellowish in color, of a friable 

 or crumbling consistence, reminding one much of dry "cottage 

 cheese" ; and imder the microscope appears as a uniformly granular 

 mass, occasionally exhibiting a few persisting fragments of the 

 original cellular elements, scattered oil droplets and crystals, and 

 when stained selects diffusely the acid stains like eosin. A moist 

 cheesy focus (which may represent an area from which the moisture 

 has not been absorbed or which may be determined from a dry 

 focus by imbibition of lymph and by liquefaction of its own sub- 

 stance) is usually not so clearly defined as a dry caseated area, is 

 paler in color, softer in consistence (pasty or mushy, like "cottage 

 cheese" mixed with milk) ; and microscopically presents the same 

 appearance as mentioned for the dry variety with the additional 

 feature that usually the fat globules and crystals are more numer- 

 ous. Caseation is especially common in tuberculous and syphilitic 

 lesions, a form of coagulation (a variety of infectious hyalin de- 

 generation) preceding the fatty change ; it however may be met 

 as a late change in a varietv of lesjons, as in atherotna, in the ^^.n- 



