214 THE NERVOUS SYSTEM OF MAN 



The afferent and efferent nerves, whether in reflex or 

 in general nervous action, never vary in the work which 

 they do. The sensory afferent nerves form the only paths 

 over which our knowledge of the outside world travels to 

 the brain. The stimuli which cause the different sensa- 

 tions, such as taste, sight, etc., have their individual paths 

 and receiving organs. This is indicated by the fact that 

 no other nerves than those of the ear are ever affected 

 when we hear. 



Reflex Action in the Frog. — The frog, like man, is able 

 to act in a definite way. If any one approaches a frog 

 while it is sitting on the edge of a pond, it jumps into 

 the water, stirs up the mud, and then returns to the shal- 

 low water near the place where it entered. The frog, in 

 this case, acts as if it, or its ancestors, had learned that 

 this is the best way to escape enemies. While this series 

 of acts is called a habit, it is really a series of reflex acts 

 which are similar to the reflex action described for man, 

 and require the same nerve structures. 



Reflex Action in the Earthworm. — If a light is flashed 

 on an earthworm at night, the worm will quickly with- 

 draw to its burrow, before it can be seized. The earth- 

 worm has no eyes, but it is able to respond to light and 

 can tell the difference between night and day. It is 

 believed that special nerve cells in the skin, which are 

 connected with the nerve ganglia, help the earthworm to 

 become aware of the light stimulus. 



Reflex Action in Hydra. — Hydra is a minute water 

 animal which has no definite nervous system, but only 

 a few nerve cells scattered through the body. As the 

 hydra waves its arms about in the water, there seems to 

 be no purpose in its motions. But if a water flea swims 

 against one of the tentacles, a part or all of the tentacles 

 at once begin to carry the flea to the mouth of the hydra. 



