The Cardinal Conditions of Infection 89 



Typhoid and cholera infections seem to 'take' place through the 

 alimentary mucous membrane, and the evidence that infection takefe 

 place by inhalation is slight. It is not known to take place through 

 the urogenital system, the conjunctiva, or the skin. '■ 



The avenue of entrance not only determines infection, but may 

 also determine the form that it takes. Thus, tubercle bacilli rubbed 

 into the deeper layer of the skin produce a chronic inflammatory 

 disease, called lupus, that lasts for years and rarely results in 

 generaUzed tuberculosis. Bacilli reaching the cervical or other 

 lymph-nodes by entrance through the tonsils, may remain localized, 

 producing enlargement and softening of the nodes, or passing through 

 them reach the circulation, in which they may be carried to the 

 bones and joints and occasion chronic inflammation with necrosis 

 and ultimate evacuation or exfoliation of the diseased mass, after 

 which the patient may recover. Bacilli entering the intestine in 

 many cases produce implantation lesions in the intestinal walls; 

 bacilli inhaled into the lung, or conveyed to it from the intestine 

 by the thoracic duct and veins, produce the ordinary pulmonary 

 tuberculosis known as phthisis or consumption. 



Inhaled pneumococci colonizing in the pharynx have been knowii 

 to produce pseudomembranous angina; in the lungs, pneumonia; 

 implanted upon the conjunctiva, conjunctivitis. In these cases 

 we can look upon the type of infection as depending upon the portal 

 through which the invading organism found its way into the tissues. 



The avenue of entrance is, for obvious reasons, less important 

 when the micro-organism is of some rapidly invasive form, whose 

 chief operation is in the streaming blood or in the lymphatics. 

 Anthrax in most animals is characterized by a bacteremia regardless 

 of the point of primary infection. Bubonic plague rapidly becomes 

 a bacteremia regardless of the entrance of the BaciUus pestis by in- 

 halation into the lungs, or by way of the lymphatics through super- 

 ficial lesions. The failure of the micro-organisms to colonize 

 successfully when introduced through inappropriate avenues may 

 be explained by a consideration of the local conditions to which 

 they are subjected. 



When they are introduced beneath the skin, bacteria are, in most 

 cases, delayed in reaching the circulation, and are in the meantime 

 subjected to the germicidal action of the lymph and exposed to the 

 attacks of phagocytes. Many succumb to these and never penetrate 

 more deeply into the body. Should any survive, they may be trans- 

 ported to the lymph-nodes and there destroyed, or, passing through 

 these barriers without destruction, and reaching the venous channels, 

 they have next to pass through the pulmonary capillaries, where 

 they are apt to be caught and destroyed. Finally, should any es- 

 cape all these defenses and reach the general circulation, it is to find 

 the endothelium of the capillaries prone to collect and detain them 

 until destruction is finally effected. The systemic circulation is 



