The entering air is also brought closely in contact with a moist surface at the narrow opening of the larynx.

That bacteria and other foreign substances can enter the lungs in spite of these guards is shown not only by the infections which take place here, but also by the large amount of black carbon deposited in them from the soot contained in the air.
Infection rarely takes place from the surface of the gullet or oesophagus which leads from the mouth to the stomach.

This is due to the smoothness of the surface and to the rapidity with which food passes over it.

Infection by the stomach also is rare, for this contains a strong acid secretion which destroys many of the bacteria which are taken in with the food.

It is found impossible to infect animals with cholera unless the acidity of the stomach contents be neutralized by an alkali.