108 INFECTION. 



ever, that infection can take place through the unbroken skin, 

 but the most minute wound or abrasion may serve as an in- 

 fection atrium. The bites of insects may furnish both the 

 wound and the infective material. Sometimes the infection 

 atrium is so insignificant as to be almost if not quite invisible. 



Conjunctive : The conjunctival mucous membrane is always 

 moist and offers a most convenient lodging-place for bacteria. 

 McFarland found a very large variety of bacteria on this 

 membrane, but no fixed species. Others claim that it is abso- 

 lutely sterile, because it is bathed continually in a secretion 

 which possesses natural germicidal powers. Congestion or 

 hypersemia increases the number of leucocytes, clogging the 

 tissues and interfering with their nutrition. Death of tissue 

 (necrosis) follows, and every barrier to infection is removed. 



Respiratory passages : These also are lined by a moist mucous 

 membrane, but do not, contrary to expectation, contain 

 many germs. The mucous lining of the nose appears to be 

 endowed with remarkable germicidal powers, so that under 

 normal conditions few bacteria ever reach the respiratory tract 

 proper. It has been estimated that 1500 bacteria are in- 

 haled every hour. Infection in tuberculosis, pneumonia, 

 diphtheria, influenza, and the exanthematous fevers occurs 

 through the respiratory tract. It is a well-known fact that 

 pneumonia usually follows a " cold," and that frequent attacks 

 of bronchitis or an attack of pneumonia predispose to tuber- 

 culosis. Plague (bubonic) pneumonia is also acquired by 

 inhalation. Bacteria may also be absorbed from the mucous 

 membranes of the air-passages and enter the adjacent lymph- 

 glands. This accounts for the finding of tubercular lymph- 

 glands at the root of the lung, and in the anterior mediastinum, 

 without the existence of pulmonary tuberculosis. 



Digestive tract : In addition to mnny non-pathogenic organ- 

 isms, the mouth may harbor also the germs of tuberculosis, 

 diphtheria, pneumonia, and the staphylococcus and strepto- 

 coccus. The saliva possesses slight germicidal power. The 

 tonsil may act as the portal of infection for tuberculosis. 

 Tuberculosis of the cervical lymph-glands probably always 

 follows tonsillar infection. An attack of pharyngitis predis- 

 poses to diphtheria. 



