296 HISTOLOGY OF THE MORBID PROCESSES. 



agency of leucocytes, which incorporate them and then pass out of 

 the necrotic area. This disintegration appears to be due partly to 

 a simple maceration or separation of the particles of the tissue, 

 partly to a solvent action exerted by the fluids in the tissues upon 

 dead organic matter. While absorption is going on there is an 

 inflammatory reaction in the surrounding tissues that still retain 

 life, which results in the formation of cicatricial tissue. This may 

 ultimately occupy the site of the necrosed tissue, or it may form a 

 capsule around a collection of fluid occupying that site, the result 

 being a cyst with a fibrous wall. 



2. ENCAPSULATION. The necrosed tissues may remain unab- 

 sorbed, or be only partly absorbed, and eventually become enclosed 

 in a capsule of new-formed fibrous tissue arising through the 

 inflammatory process mentioned above. In this case the necrosed 

 mass becomes desiccated through absorption of its fluid constituents, 

 and may eventually be infiltrated with lime-salts, calcified. 



3. GANGRENE. This occurs in two forms, distinguished as dry 

 and moist gangrene. 



Dry gangrene is due to the desiccation of dead tissues that are 

 exposed to the air. The tissues become discolored, owing to changes 

 in the coloring-matter of the blood, and shrink, the skin assuming 

 the appearance of parchment. After a time the dead mass is cast 

 off by the formation of granulation-tissue from the neighboring 

 living tissues. 



Moist gangrene is the result of putrefactive changes in dead 

 tissue, due to infection with bacteria causing decomposition. The 

 parts are discolored, swollen, moist, and often contain bubbles of 

 gas having a foul odor. The gangrenous part may here also be 

 cast off as the result of the formation of granulations, but the 

 gangrenous process may spread before it can be checked by an 

 inflammatory demarcation, the products of decomposition having 

 a poisonous effect upon the neighboring tissues that leads to necrosis 

 and prevents the development of granulation-tissue. 



4. SUPPURATION. If the dead matter contain pyogenic micro- 

 organisms, they exert a peptonizing action upon the necrotic mass, 

 causing it to liquefy. At the same time they excite a purulent 

 inflammation in the surrounding tissues which leads to the forma- 

 tion of an abscess or an ulcer. 



In those cases of necrosis in which the necrosed tissues are not 

 speedily absorbed the dead mass is known as a " sequestrum," and 



