192 PRINCIPLES OF VETERINARY SCIENCE 



determining causes produce visible changes in the tissues that 

 are termed lesions. Generally speaking lesions are the result 

 of disturbed nutrition, or mechanical, thermic, electrical, or 

 chemical injury. While it is doubtful if predisposing causes in 

 moderate action are capable of producing a distinct lesion of 

 sufficient importance to be termed disease, it is certain that 

 actual disease will be produced by any of the following determin- 

 ing causes: 



(1) Traumatisms comprise all mechanical causes that damage 

 the body structures, or in any way disturb the functions of a 

 part of the body. Under this head are classed sprains and strains; 

 cuts, tears, and bruises; excessive pressure and rubbing from an 

 improperly fitted collar or saddle, which produce injury first to 

 the subcutaneous tissues and then to the surface of the skin 

 (galls) ; electric shock or burns either from contact with a charged 

 object or from lightning. A burn of any kind is a traumatism; 

 likewise a frostbite. When well-marked, either of these injuries 

 leads to death of the affected part. Light is yet another physical 

 cause of disease for undue exposure tojlight may result in affections 

 of the skin or eyes. A more complete discussion of traumatic 

 injuries and their treatment will be found in the chapter on 

 Wounds. Mention should be made of surgical operations in 

 this connection, as they are often the exciting cause of serious 

 disease especially when improperly performed and when the sur- 

 gical-wound is not given needed after-treatment. 



(2) Poisons are substances that destroy life or impair the 

 functions of one or more of the body organs. They may be 

 separated into three classes, as follows — animal poisons, like snake 

 venom, which is highly toxic; vegetable poisons, like opium and 

 strychnin; mineral poisons, like arsenical salts, caustic soda, and 

 silver nitrate. Some mineral poisons exert their destructive 

 influence by causing a coagulation of the tissue-juices; others, by 

 causing a liquefaction of the tissue-cells. Many animals seem 

 to be endowed with an instinct which teaches them to avoid most 

 poisons. Some poisons, such as snake venom, are entirely harm- 

 less when taken into the body by way of the mouth, but are 

 deadly if introduced through the skin. Other poisons are most 

 active after gaining access to the blood or some other tissue. 

 Still other poisons are harmless until their composition has been 

 changed in the animal body, thus calomel is probably converted 



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