178 Necrosis. 



Hirschfeld) observed a destruction of the gray matter as a result 

 of withholding the blood supply for a single hour in case of the 

 spinal cord. Interference ^Vith the venous outflow, if marked 

 enough to cause stasis in the capillaries, is also followed by nu- 

 tritive disturbances which terminate in gangrene. 



The vitality of the tissues may also be overcome by chemical 

 and thermic influences producing coagulation or liquefaction of the 

 proteids and thus either directly or indirectly rendering the pro- 

 toplas^i incapable of appropriating nutrition and giving off its 

 metabolic products, as well as interfering with the interchange of 

 gases (burns, freezing, poisons like the acids and alkalies, the 

 digestive juices, bacterial poisons). Evaporation of moisture, as 

 in case of evisceration, causes necrosis of the exposed organs partly 

 by drying the protoplasm of the cells, partly by desiccating and 

 stagnating the blood and lymph. [An important adjuvant cause 

 of necrosis is also seen in the destruction of innervation of a part, 

 as may be observed in the massive and fulminating necrotic changes 

 met with in cases of lesions of the spinal cord at points of pressure 

 below the level of the spinal injury, the well known gangrenous 

 decubitus of such individuals. In some measure here, disregarding 

 the important pressure factor, the necrosis may depend upon 

 vascular changes due to the ner\'>ous lesion ; but there are reasons 

 for believing a more direct cellular failure results as well, from 

 the loss of the nervous trophic control.] 



As a general rule the occurrence of necrosis is to be attributed 

 to a dual influence, the causative lesion having both a direct destruc- 

 tive power upon the tissue and at the same time interfering with 

 the circulation of its fluids. 



The characteristics of necrotic and dying parts vary with their 

 structural peculiarities, the proportion of moisture in the organ, 

 the relation with the uninvolved parts, and external influences. 



Dense structures like bones and teeth are subject to a simple 

 necrosis; these and the dense elastic tissues are extremely resistant 

 to physical and chemical agencies, retaining long after complete 

 death their form and general appearance as in life. Soft tissues 

 also, like the epithelium of the stomach, if killed by poisons having 

 disinfecting power (carbolic acid poisoning), may remain without 

 material change, with but little alteration of form, just as an'a- 

 tomical specimens preserved in alcohol or formaldehyde solutions. 

 In such instances the necrosis is primarily recognized by the reac- 

 tion in the surrounding healthy structures in their effort to isolate 

 the dead part (sequestration). 



