354 THE PRESERVATION OF HEALTH 



515. Poisons are usually divided into two classes : cor- 

 rosive or irritant poisons, and narcotics. 



Corrosive poisons cause great changes in the tissues. 

 In this class are included all those which affect the skin, 

 causing inflammation and sometimes destruction of the 

 tissues ; those which act upon the mucous membranes of 

 the alimentary canal, giving rise to inflammation, nau- 

 sea, vomiting, pain, and purging ; and those which act 

 upon the mucous lining of the respiratory organs. These 

 last are usually gases. A poison to affect the system must 

 be in liquid or gaseotis form, but solid poisons may be 

 quickly dissolved by the fluids of the mouth or the stom- 

 ach, or the exudations from the broken surface of the 

 skin, and the solution becomes dangerous. Some solid 

 poisons, however, dissolve so slowly that they may pass 

 through the alimentary canal and be expelled from the 

 body without doing harm. Those poisons which enter the 

 blood, and by causing alterations in it, or by circulating 

 in the blood, injuriously affect various organs, belong to 

 the class of irritants. The symptoms resulting differ as 

 one organ or another is most affected. 



516. Narcotics do not produce marked tissue changes, 

 but affect chiefly the nervous system. The results vary 

 greatly. Sometimes convulsions, cramps, delirium ; some- 

 times depression, sleep, or stupor may be caused. Nar- 

 cotics work their effects more slowly than do the irritants, 

 and the particular symptoms which appear depend so much 

 upon the special susceptibility of the individual that diag- 

 nosis is often very difficult. Some poisons which produce 

 narcotic effects are also corrosive'to the tissues, so that the 

 classification into irritants and narcotics is not an absolute 

 and scientific one. 



517. Treatment. In case of poisoning, the object of 



