IMMUNITY AND SUSCEPTIBILITY 689 



the dead and desquamating epithelial cells. The skin is impermeable 

 to these microorganisms when it is unbroken in its normal state. 

 Experiments have been performed to detetmine whether the skin is 

 normally permeable to bacteria. Bacteria have been rubbed into the 

 skin and have produced infection but in these instances the skin has 

 been abraded by the mechanical irritation. Bacteria may infect the 

 sudoriferous and sebaceous glands and their ducts, in case the metabolic 

 activity of these structures is disturbed. The ducts and the glands of 

 the skin are protected ordinarily ^by a flow of the secretions. In case 

 the flow of the secretions is decreased and the orifices of the ducts 

 contracted as in cold weather, while bacteria find it more difficult to 

 pass down than before, they occasionally do produce an infection. 

 When a hair follicle is diseased and the shaft contracted or perhaps 

 dropped out, bacteria may pass down and produce an infection. B. 

 tetani of tetanus or lockjaw frequently passes through the skin by 

 means of deep penetrating wounds. The same is true of some other 

 pathogens. 



In case bacteria are successful in permeating the skin either directly 

 or by means of cutaneous orifices, they are usually able to set up a 

 marked inflammation of these structures and produce necrosis of the 

 epithelium. It is in this way that pustules, boils, carbuncles, and 

 various forms of cellulitis are produced. The secretions of the sebace- 

 ous glands are not germicidal but are perhaps slightly antiseptic due 

 to the salts which are contained therein. Furthermore, as soon as the 

 serum from the blood is extravasated there may be slight germicidal 

 action on the bacteria infecting the skin. The soluble toxins of bacteria 

 cannot be absorbed through the unbroken skin. 



The Subcutaneous Tissue. — In case the bacteria are successful in 

 permeating the skin and penetrating the subcutaneous connective 

 tissue, again various protective mechanisms show themselves. This 

 resistance is due to a very rapid production of new connective tissue 

 which serves to mechanically limit the infection. It is due, further- 

 more, to the germicidal action of the serum, the mechanical and germi- 

 cidal action of the fibrin and the phagocytic activity of the leucocytes. 

 These various factors will be discussed subsequently in connection 

 with the phenomena and the protective mechanisms of inflammation. 



The Exposed Mucous Membranes of the Body. — The exposed mucous 

 membranes of the body usually are covered with a variety of bacteria, 

 44 



