THE RELATIOX OF BACTERIA TO DISEASE 149 



their development, as in the case of anaerobic bacteria. Not infrequently 

 the tetanus bacilli or spores would not be able to develop in wounds 

 were it not for the presence of aerobic bacteria introduced with them. 

 This is shown outside the body, where tetanus bacilli will not grow in 

 the presence of oxygen unless aerobic bacteria are associated with 

 them. Again, it is found that the association of one variety with another 

 mav increase its virulence. Streptococci are stated to increase the viru- 

 lence of diphtheria bacilli, but here it is probably the loss of resistance 

 of the tissues because of the streptococcic poison. On the other hand, 

 the absorption of the products of certain bacteria immunizes the body 

 against the invasion of other bacteria, as shown by Pasteur that 

 attenuated chicken-cholera cultures produce immunity against anthrax. 



Ability of Bacteria to Penetrate the Skin and Mucous Membranes. THE 

 SKIN. The unbroken skin is a great protection against the penetra- 

 tion of micro-organisms. When they do penetrate it is through the 

 glands, or more often through some unobserved wound. Soluble 

 vegetable poisons, such as aconite and bacterial toxins, are not 

 absorbed. 



There is an apparent exception to the above statements in the fact 

 that the pyogenic staphylococci and sometimes the streptococci exist 

 upon the skin or in it between its superficial horny cells, some excep- 

 tional circumstances, such as wounds or burns, being required to allow 

 the organisms to penetrate deeper. The cutaneous sweat glands, and 

 the hair follicles with their appended sebaceous glands, may allow 

 entrance of infection, as various incidents may lead to the introduction 

 and retention of virulent micro-organisms. When this occurs the 

 retained products may lead to necrosis of the epithelium and thus 

 allow the bacteria to penetrate to the deeper tissues. The secretion 

 of the sebaceous glands appears not to be bactericidal, but the acidity 

 of the perspiration renders it slightly so. 



SUBCUTANEOUS CONNECTIVE TISSUES. Many bacteria cannot de- 

 velop in the connective tissues and others produce a milder infection 

 there than elsewhere. Others develop readily and cause infection. The 

 rapidity of development of new connective tissue and the bactericidal 

 properties of the lymph are the main known hindrances to infection. 



THE Mucous MEMBRANES. The moist condition of the surface of 

 the membranes aids bacterial multiplication. Mucus is only slightly 

 bactericidal for some bacteria and not at all for others. Bacteria, such 

 as the pneumococci and streptococci, remaining in it become somewhat 

 attenuated. The conjunctival mucous membranes are protected by 

 the cleansing produced by the flow of the lacrymal secretion and by 

 its slight germicidal action. In infancy the membranes are readily 

 infected by gonococci and later by pneumococci, the Koch-Weeks 

 bacillus and others. Many soluble poisons and toxins are absorbed. 

 The mucous membranes of the nasal cavity are somewhat cleansed by 

 the nasal secretion, which is feebly bactericidal. The deeper portions 

 of the nasal cavity are usually the seat of streptococci and other 

 bacteria. The mouth in a person in health is cleansed by the feebly 



