THE NERVOUS SYSTEM 



217 





more of their special products, as is the case with the gland cells 

 or with the white corpuscles. Most striking is the fact that 

 some cells have specialized in receiving disturbances and in 

 transmitting them — the nerve cells. 



A flash, a sound, a push, any occurrence to which protoplasm 

 is sensitive is called a stimulus. The contraction, the turning 

 aside, the scream, or 

 whatever it is that the 

 organism does when it 

 is stimulated, is called 

 the reaction or the re- 

 sponse.^ In the highest 

 animals, like ourselves, 

 we recognize three dis- 

 tinct types of reaction : 



1. Muscle. A stim- 

 ulus may set up move- 

 ments. Some of these 

 we can see in the limbs, 

 the trunk, the face, and 

 so on ; others take place 

 in the heart, in the 

 walls of the intestine, and so on (see Fig. 108). A sudden noise 

 may startle one so that the whole body is visibly shaken : in 

 another case one may keep his outward composure, yet react 

 by a change in the heartbeat. 



2. Gland. A stimulus may set up reaction in one or more 

 glands. The odor of well-liked food starts the salivary glands 



^ Note that the relation between the stimulus and the reaction is not of the 

 same kind as that between the pushing of an object and its sliding or falling. 

 In the latter case the object moves in direct proportion to the strength of the 

 push that was applied to it. In a living organism the stimulus may represent 

 a very slight amount of energ\', while the reaction may involve a very great 

 amount of energy. A slight touch on the sole of a man's foot may bring forth 

 a violent kick. The relation between stimulus and reaction, so far as the 

 amount of energ:>' is concerned, can better be compared to the relation between 

 the pressure on the trigger and the explosion of the gun. 



Fig. 108. Contraction of a muscle 



The movement of an organ, as the forearm, is 

 brought about by the contraction of a muscle. The 

 mass of muscle cells becomes shorter and thicker, 

 the parts to which its ends are attached being 

 brought closer together. The movement of the 

 muscle is set off by a nerve, not shown 



