68 VITAL ANIMAL LIFE PROCESSES 



Sensory Reactions 



The processes of digestion, respiration, and excretion usually take 

 place more or less automatically within an animal's body. In addition, 

 there are many reactions which come about as a direct adjustment to 

 external stimulations. These are vital reactions which every animal 

 must make if it is to survive the constantly changing conditions of en- 

 vironment. In the complex animals there is an elaborate series of 

 sense organs and nerves which detect external stimulations, and muscles 

 all over the body which respond to nerve impulses and make the proper 

 responses. Not a day passes that we do not react to some dangerous 

 environmental situation to save ourselves from serious injury or death. 

 A hand that accidentally touches a hot stove is quickly removed as the 

 sense organs in the skin are stimulated by the heat, and the muscles 

 of the arm quickly respond to draw the hand away. We jump to safety 

 at the sight of a rapidly approaching automobile. The sight and odor 

 of good food stimulate our desire to eat, and our muscles carry the 

 food to our mouths. We could multiply such examples of stimulus 

 and response which under usual circumstances lead to our survival. A 

 human being could not survive long without some senses to detect 

 changes in the outside environment and muscles to respond to these 

 changes. Of course, many people live with one or two of their senses 

 lacking, sight and hearing most commonly, but some senses must re- 

 main. Also paralysis may incapacitate some muscles, but certain vital 

 muscles must retain their power to function. When vital nerve centers 

 are destroyed by disease or injury, death results. 



In the animals which are not so highly developed, there is a cor- 

 respondingly lesser degree of development of the sensory reactions, but 

 they are present even in the one-celled animals. Studies of one-celled 

 animals show that they can detect the presence of food and make an 

 effort to obtain it ; they can avoid substances that are unfavorable to 

 them ; they can respond to changes in temperature and light intensity. 

 It is remarkable how many activities can be performed by one cell 

 when it is the only cell of an animal body. As we survey the animal 

 kingdom, it will be interesting to note how the various senses are de- 

 veloped, along with the ability to respond to the stimulation of these 

 senses. 



Transportation 



All animals are faced with the problem of moving materials from 

 one part of the body to another. Among the smaller animals living 



