132 ELEMENTARY BIOLOGY 



amount of it will kill a person or a rabbit.^ In experiments 

 this substance was given to rabbits in very small quantities, — 

 a fraction of the quantity that it would take to kill. After a 

 few days the animals were given a little more. The dose was 

 gradually increased until the animals had become so accus- 

 tomed to the substance that they could stand several times the 

 ordinary fatal dose. The arsenic acts upon the protoplasm of 

 the nerves or muscles in such a way as to put the animal in 

 a state of tonus — that is, the way one feels when one is "all 

 on edge," ready to jump or scream on the slightest provoca- 

 tion. The rabbits treated with arsenic thus became extremely 

 sensitive to the slightest disturbance ; they would jump on 

 hearing the faintest sound, or on seeing the slightest move- 

 ment, or the passing of a shadow. But the most curious result 

 was that after animals had been treated with the poison in this 

 way for a considerable time, it ivas impossible for them to live 

 ivithoiit it. If the drug was omitted from their daily rations, 

 they quickly died. 



162. Action of drugs. It seems that when any substance out 

 of the ordinary gets into the protoplasm, it may behave in one 

 of three ways. Either it remains without any effect on the 

 protoplasm, or it combines chemically with one or more of the 

 substances in the cell. In the latter case the partial removal 

 of the protoplasm material either depresses the action of the 

 living substance or makes it go faster. 



163. Stimulants and narcotics. Anything that changes the 

 condition of the protein, or removes fat, or hastens oxidation 

 — or stops oxidation — must modify the action of the living 

 protoplasm. A substance that makes protoplasm work harder 

 or faster is called a stimulant. One that slows or depresses 

 the activity of protoplasm is called a narcotic. Both stimulants 

 and narcotics are therefore poisons, since the final effect of 

 either may be to stop the action of protoplasm permanently. 



1 Strangely enough, a child can stand more arsenic than an adult. 



