HEALTH AND DISEASE 337 



the whole body. The dangerous germ may, indeed, be 

 caught by the moist surface of the mucous membrane and 

 removed before it has done harm; for it is one of the 

 functions of the moist lining of the winding nasal passages 

 to strain out the irritating and disease-bearing particles 

 from the air, and the ciliated epithelium of the lower 

 air passages also affords protection in the same way. 

 Expired air is practically free from germs and other dust. 

 What we call " taking cold " is now believed to be due to 

 the poisonous action of bacteria fastening upon the mucous 

 membrane where the cells have been injured by cold or 

 in some other manner. Finding food at the injured spot, 

 the bacteria multiply and cause inflammation of the air 

 passages and general discomfort, sometimes resulting in 

 disease. 



487. It has been found that a large number of bacteria 

 of several varieties are always present in the human mouth. 

 The saliva has the power of destroying a limited number 

 of the harmful kinds, but others remain to multiply. 

 Certain sorts cause decay of the teeth (where the enamel 

 is defective). Others may make their way into the lungs 

 or the stomach. As they grow they manufacture from 

 the albumin which they decompose, certain poisons, called 

 toxins. These may enter the circulation and carry the 

 deadly influence to all parts of the system. If the stomach 

 is healthy and the gastric juice normal, many dangerous 

 germs will be destroyed in the stomach. It is even shown 

 by experiment that those of cholera and typhoid fever are 

 quickly killed by the gastric juice. Other germs have 

 greater resisting power and are able to multiply in the 

 stomach, causing dyspeptic symptoms or specific disease. 

 Some forms of bacteria feed upon and destroy other forms. 

 Many sorts escape the destructive action of other germs 



