Causes of Xccrosis. 177 



of degeneration it is also indicated by the term necrobiosis. Be- 

 tween the degenerations (the retrograde metamorphoses) and the 

 necroses it is impossible to draw any sharp line of difference. (There 

 are in addition a number of other expressions in use for the special 

 types of necrosis by which the death of a part in the living body is 

 seen to occur, which are often emplo\ed indifferently as synonyms — 

 vide infra). Even in normal life in the interchange going on in 

 the course of the physiological processes there is a certain amount 

 of the cellular material which is being broken down in the tissues, 

 consumed and replaced by new cells. The epidermal cells of the 

 skin are continually undergoing keratinization. later desquamat- 

 ing as dead structures, and newly formed cells from the stratum 

 Alalpighii take their place to undergo in their turn the same 

 change. In the same way the epithelium of various glands die 

 and contribute their own substance to their secretions, as the cells 

 of the sebaceous and mammary glands. The blood cells are also 

 perishable elements, some being all the time in course of disintegra- 

 tion and death, the formation of new cells continually proceeding 

 in the h?emopoietic parts (bone marrow and lymph glands) to 

 compensate for this loss. Other physiological instances of local 

 death are also seen in the loss of the milk teeth and in the nnunmi- 

 fication of the umbilical cord. 



Pathological conditions capable of causing local death are 

 usually such as interfere with the nutrition of the cells and tissues^ 

 and render the metabolic interchange between them and their 

 surroundings impossible. The most common is some interruption 

 of the blood supply : as thrombosis and embolism ( both of which 

 occlude the vascular lumen). ru])ture of the vessels, separation of 

 the cells or tissues from their sources of supply (traumatic, or the 

 undermining of the skin in case of subcutaneous suppuration), or 

 compression of the tissues which interferes with the entrance of 

 blood to the part. If the blood is unable to gain access to an 

 organ or some portion of an organ, no more than mere microscopic 

 parts of it can be maintained by the lymph ; with the exception of 

 the cornea and the cardiac valves, for which the lymphatic fluid 

 is the principal source of nutrition, no organ can withstand a pro- 

 tracted interruption of its blood supply. This is especially true 

 of gland cells ; according to Litten two hours are sufficient after 

 ligation of the renal arteries in rabbits to cause necrosis of the 

 renal epithelium, even though after removal of the ligatures the 

 circulation be re-established. Ehrlich and Rrieger 1 cited from Birch- 



