IMMUNITY AND SUSCEPTIBILITY 691 



the trachea and bronchi into the lungs and through the oesophagus into 

 the stomach and intestines. 



The Lungs. In case infectious microorganisms pass down the 

 trachea and bronchi they meet first with the obstruction of the mucus 

 which is secreted upon the surfaces of these tubes. In addition, ciliated 

 epithelial cells are present and serve to cleanse the surfaces from 

 microorganisms as in the nose. Occasionally microorganisms lodge 

 along the trachea and the bronchi and produce slight irritations which if 

 left undisturbed may immediately produce serious infections. How- 

 ever, the leucocytes from the neighboring bronchial and mediastinal 

 lymph glands pass through the walls of the trachea and bronchi, ingest 

 the microorganisms, carry them back to the glands and in a majority 

 of instances destroy them. Occasionally, however, leucocytes contain- 

 ing virulent microorganisms get into the lymphatic circulation and these 

 are carried by the diffusion currents in the lymph vessels down to the 

 alveoli of the lungs and here may cause inflammations of a more or less 

 serious character. It is probable that the Strept. pneumonia is very 

 frequently carried to the alveoli of the lungs in this way. Without 

 doubt, microorganisms cannot be directly inhaled through the air 

 passages into the alveoli of the lungs during an ordinary inspiration, 

 but it has been shown that in forced inspirations, such as those attend- 

 ing upon coughing, hiccoughing, sneezing and sighing that they may be 

 so carried. 



The Stomach. In case the microorganisms pass down the oesophagus 

 into the stomach, they immediately come into contact in the normal 

 organ, with the gastric juice, which contains the hydrochloric acid in 

 such concentration that it is at least antiseptic if not germicidal. 

 In case the functional activity of the stomach is disturbed and the 

 hydrochloric acid is diminished in amount, microorganisms may grow 

 in the stomach to a limited extent. Furthermore, in case all the 

 particles of food are not thoroughly broken up in the stomach, bacteria 

 which may be contained within these particles may pass through the 

 stomach into the intestine. 



The Intestines. In the intestines the microorganisms come into 

 contact with the alkaline pancreatic juice which is slightly antiseptic 

 and with the bile which is antiseptic and in certain instances bacteri- 

 cidal. They find no particularly favorable conditions for growth in the 

 upper part of the small intestines under normal conditions. Here 



