158 PRINCIPLES OF VETERINARY SURGERY 



THEORIES OF IMMUNITY. 



Before discussing the theories of immunity, we shall 

 take up the natural defenses of the body. 



Every time bacteria come in contact with a susceptible 

 animal, infection does not necessarily follow, otherwise the 

 animal race would soon be destroyed. Most all pathogenic 

 bacteria can be isolated from the hide and hoof of animals 

 — streptococcus, staphylococcus, tetanus, etc. — while in the 

 mouth are normally found such germs as pneumococcus 

 and influenza bacillus ; in the intestinal tract, the colon bacil- 

 lus, or the bacillus dysenterije. The body has natural de- 

 fenses which prevent the development of these bacteria. 

 First, the 



Skin constitutes a covering of great importance as a 

 means of protection against microbial invasion. In all the 

 domestic animals it is a prolific seat of bacterial flora, which 

 seize every favorable opportunity to attack the animal, pro- 

 ducing abcesses, phlegmons, or even general infection of 

 blood and tissue, as in septicsemia and pyaemia. Normally, 

 the skin can resist these attacks by the constant production 

 and expulsion of its corneal cells. This continued exfolia- 

 tion of the cells of the deeper layers of the epidermis results 

 in a simultaneous eviction of such micro-organisms as are 

 living on them. The connective-tissues of the skin, also the 

 hair, horns, and hoof, are normally able to protect them- 

 selves against bacteria by reason of their impenetrability. 

 All these tissues have the service of the phagocytes, which 

 wander about everywhere, ever ready to pick up foreign 

 invaders. 



Mucous membranes are able to resist bacteria by the 

 throwing-ofT of their cells, which take the germs with them, 

 or by the antiseptic protection of the mucus and saliva 

 with which the germs come in contact. These secretions 



