104 INFECTION AND IMMUNITY. 



rubbed into the skin. This is also true of most 

 chemical poisons which do not have a corrosive 

 effect, although by prolonged contact (lead) or 

 rubbing (mercury) a certain amount of absorp- 

 tion may be induced. 



In infections of the mucous membranes, with 

 surfaces. their soft epithelial covering and moist surfaces, 

 micro-organisms are often able to grow through 

 into the underlying tissue, resulting in either a 

 local inflammation, a general infection through 

 the medium of the lymphatics or capillaries, or a 

 general intoxication through the absorption of 

 toxins. A mechanical defect in the surface may 

 not be necessary for this penetration, the exten- 

 s i n taking place by contiguous growth, which, 

 Growth, however, is surely favored by any toxic and desqua- 

 mating effect which the organism or its toxin may 

 have on the epithelium. Streptococci which reach 

 the crypts of the tonsils readily cause necrosis of 

 the surface epithelium, and this defect would seem 

 to facilitate deeper invasion. 



Diphtheria. The growth of the diphtheria bacillus is limited 

 in the main to the superficial layers of the tissues 

 which it attacks. As the organism penetrates the 

 epithelial layer, possibly by direct growth, the toxin 

 which it secretes causes necrosis of the adjacent 

 cells. This process continues until the vascular 

 tissues are reached, resulting in the formation of a 

 false membrane. However, the onset of fever 

 before the formation of the membrane, and the 

 occurrence of diphtheria without membrane forma- 

 tion, show that the false membrane, i. e., the necro- 

 sis of the surface, is by no means a prerequisite for 

 the absorption of the toxin. A severe invasion of 

 the body by the bacillus does not occur, although 



