242 AGRICULTURAL BACTERIOLOGY 



days to several months is called the period of incubation 

 of the disease. The changes in the various tissues due to 

 the action of the organism are called the lesions of the dis- 

 ease. With some diseases they are very characteristic, in 

 others not. 



The external defenses of the body. There are many 

 means by which nature has sought to protect the body 

 against the invasion of microorganisms. The surface of the 

 body is covered by the skin, and all of the cavities of the 

 body that are in contact with the exterior are provided with 

 a mucous membrane. The bacteria normally gain entrance 

 with but few exceptions through these protective mem- 

 branes, only as they are injured. The mucous membranes 

 are always bathed with the products of glandular activity, 

 which possess a more or less marked germicidal or antiseptic 

 action. By reason of this many of the organisms that come 

 in contact with these fluids are thus destroyed. Wounds in 

 the mouth and in the intestine must of necessity frequently 

 occur, especially with animals that feed on coarse, dry 

 fodder. Yet a harmful effect from such a source is rarely 

 noted. The lungs and air passages are constantly exposed 

 to dust laden with adherent bacteria. These foreign bodies 

 are removed by the action of the cilia of the cells lining the 

 air-passages. The hairlike appendages are constantly in 

 motion and tend to move any foreign particle outward. 



The internal defenses. After the microorganisms have 

 invaded the tissues, their development can not go on un- 

 hampered, for the body has a number of internal defenses 

 that must be overcome before growth and disease produc- 

 tion can occur. An animal is said to be immune to a dis- 

 ease when it resists the development of the organism, or is 

 not injured by the poison that the organism produces. 

 Various explanations have been offered to explain the im- 

 munity of animals. The white blood corpuscles possess 



