196 PRINCIPLES OF VETERINARY SCIENCE 



there are many things that are not satisfactorily explained or 

 well understood. 



Health is that ideal state of the body in which all organs are 

 working in harmony and in which the body is best able to resist 

 attack by disease. It can be maintained permanently only 

 when a perfect balance exists between feed and work. The 

 body, however, has a wonderful adaptive capacity so that it is 

 able to maintain its normal structure and functions in spite of 

 adverse conditions. 



The body possesses two classes of defensive mechanisms against 

 disease. One class operates externally, the other internally. 



I. The External Defenses. — Under this heading are classified 

 the intact skin and the intact mucous membranes. Both are 

 composed of epithelial cells which are capable of excluding most 

 microorganisms and of serving as efficient defenses against all 

 invaders from those surfaces of the body that come in contact 

 with the air. 



(1) The intact skin, as has been learned, serves mechanically 

 to protect the outer surfaces of the body. When normal it is 

 impervious and highly resistant even to poisons and chemicals. 

 On those parts of the body most exposed to injury it is very thick 

 and tough. Its power of rapid replacement when injured or 

 destroyed renders it well adapted for an external defensive 

 mechanism. 



(2) The intact mucous membranes are extensions of the skin 

 and serve to prevent invaders of a bacterial nature from entering 

 the tissues which they cover. These membranes are normally 

 always moist and velvet-like from mucus, the secretion which 

 bathes their surfaces and keeps them from erosion when they 

 rub together, as often occurs on account of their situations. 

 The mucus hinders the penetration of bacteria and washes them 

 away. In the chapter on respiration an account of the protection 

 afforded by the ciliated cells lining the respiratory passages will 

 be found. Other secretory products that protect are those from 

 the liver and the salivary, the gastric, the lacrimal, and the 

 serous glands. Of especial note are bile and the acid gastric 

 juice, both of which possess marked antiseptic and bactericidal 

 properties. 



II. The Internal Defenses. — The agencies that afford internal 

 protection to the body against disease are all concerned with 



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