THE CONTROL OF THE BODY 



CHAPTER XXII 



THE NERVOUS SYSTEM 



Questions. 1. What is the use of pain ? 2. Would there be any harm 

 in killing the nerves in the teeth ? 3. Are there any animals that have no 

 nerves ? 4. Do animals feel pain as we do ? 5. Are there any activities 

 in the body that we cannot control? 6. What is the use of the funny 

 bone? 



177. Irritability. The irritability of protoplasm (see sec- 

 tion 43) is the basis of our brain and nerves. There are mix- 

 tures of substances (non-living) that are exploded by a beam of 

 light. The distinctive thing about the irritability of protoplasm 

 seems to be that instead of exploding and going to pieces when 

 disturbed, as dynamite does, for example, protoplasm brings 

 about a change that on the whole tends to preserve it from 

 further injury. It may shrink away from the point of disturb- 

 ance; it may bring about a chemical change that counteracts 

 the disturbance ; it may rearrange itself so that the disturbance 

 does no damage. But it cannot be said that protoplasm meets 

 every disturbance in a suitable way, for it is possible to poison 

 or kill protoplasm. We can only say that in general what proto- 

 plasm does in response to what happens to it is more or less 

 suited to help the organism or to save it from injury. 



178. Specialized irritability. With the division of labor in 

 many-celled plants and animals, there is also a specialization of 

 irritability. In our own body, for example, certain cells or 

 tissues respond to disturbance by a rather sudden contraction. 

 Others show that something has happened by increasing the 

 amount of chemical change going on in them, and secreting 



216 



