IRRITABILITY IN MAN 



205 



Water, also, acts as a stimulus; sometimes it is 

 stronger than gravity and causes a turning of roots 

 from their normal direction toward the water supply. 



Irritability in man. In 

 order to understand the 

 reactions of man, the 

 mechanism that brings 

 about the reactions must 

 first be studied. This 

 mechanism is called the 

 nervous system. It is 

 often compared to a tele- 

 phone system. An expert 

 operator at a switchboard 

 quickly brings different 

 rooms in a building, dif- 

 ferent homes in a city, 

 different cities, and even 

 different nations into communication by means of messages sent 

 over various connecting wires. In a similar way, the various 

 organs of our body are made to work together by means of nerves 

 that are brought into connection by means of nerve centers. 

 There are two groups of structures composing the nervous 

 system : (1) the central nervous system and (2) the sympathetic 

 or autonomic nervous system. These two systems are intimately 

 connected with each other. 



Protection of the central nervous system. The central nervous 

 system consists of the brain and the spinal cord. The brain is 

 covered and well protected by three membranes which separate 

 the skull from the skull cavity. These membranes secrete the 

 cerebro-spinal fluid and contain blood vessels which transmit 

 blood to all parts of the nervous system. If glancing blows strike 

 the skull, the movable mat of hair and skin tend to weaken the 

 force of the blow. In infants, the skull bones are not completely 

 joined together and, consequently, their brains are not as well pro- 

 tected as the brains of adults. However, by the time the child is 



