414 A YEAR IX SCIENCE 



A number of days elapse from the time the germs 

 enter the body until they exist in sufficient numbers 

 for the disease to appear. This time is known as the 

 incubation period. The length of this period varies with 

 the kind of disease. For measles it is nine days, scarlet 

 fever two to four days, typhoid fever two weeks, and 

 grippe from one to five days. 



How the body destroys germs. In spite of these 

 innumerable and active enemies/ the body is generally 

 well able to defend itself. It has at least three means 

 of destroying germs; by the white Hood corpuscles, 

 by substances called germicides, and by other .sub- 

 stances called antitoxins. 



1. White corpuscles. We have already referred to 

 the method by which the white blood corpuscles destroy 

 germs. They are present not only in the blood, but 

 also in all parts of the body. If bacteria are present, 

 they surround and * destroy them. The.y are always 

 present in great numbers around a wound. 



2. Germicides. In the blood there are certain 

 unknown substances formed which destroy germs. 

 They are called germicides. If these substances are 

 present in sufficient quantities in the blood, they kill 

 the disease germs and consequently protect the body 

 from disease. 



3. Antitoxins. The injurious effects of disease are 

 sometimes the result of poisons, toxins, which the 

 germs produce. There is sometimes produced in the 

 body a substance called antitoxin (anti, against). This 



