be indifferent to the action of morphin, and rabbits are insensitive to the 

 alkaloid atropin, or belladonna. 



Members of the same species also differ greatly among one another. 

 Some persons are more susceptible than others to the effects of tobacco or 

 alcohol; some more susceptible to the specific poisons of particular kinds of 

 bacteria. How various poisons act upon the organism and how they can be 

 counteracted are the problems of a special study — toxicology. In the last 

 few decades we have learned a great deal about how the body reacts to 

 foreign substances of various kinds. 



How Is Protoplasm Influenced by Foreign Substances? 



Getting Used to Changed Conditions There are many kinds of fish 

 that live in salt water only, and there are many kinds that live in fresh 

 water only. Some species, however, such as the salmon and eel, spend part 

 of their lives in the ocean and part in fresh water. Still, if we took one such 

 fish out of the ocean and placed it in fresh water, it would soon die. Or 

 if we took one from fresh water and put it into salt water, it would soon 

 die. But if we slowly dilute sea water, or gradually concentrate the salt in 

 fresh water, we can keep some fish alive now in one medium and now in 

 the other. 



In a case of this kind we say that the animal "gets used" to living in the 

 new conditions. This illustrates a pretty general fact about protoplasm, or 

 about living things. Living things can get used to new conditions of tem- 

 perature or of light or of chemicals or of food. This does not mean that 

 every living thing can come to live in any kind of surroundings whatever. 

 That is not true. Birds cannot get used to living in water; fish cannot get 

 used to living in the air. Plants and animals cannot get used to living with- 

 out proteins or without salts. But we can all change our conditions of living 

 to a certain degree or in certain directions and still remain alive. 



Habit-Forming Poisons Arsenic is poison to all kinds of protoplasm. 

 It is used in fighting many kinds of insects and many kinds of fungi. A 

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

 substance was fed to rabbits in very small quantities — a fraction of the quan- 

 tity enough to kill. After a few days the animals were given a little more. 

 The dose was gradually increased until the animals could stand several 

 times the ordinary fatal dose. The arsenic acts upon the protoplasm of the 

 nerves or muscles to put the animal in a state of tonus, or stretch — that is, 

 the way one feels when one is "all on edge", all set to jump or scream on the 

 least provocation. The treated rabbits thus became extremely sensitive to 

 the slightest disturbance. They would jump on hearing the faintest sound, 



■^Strangely enough, a child can tolerate more arsenic than an adult. 



232 



