THE REGULATING MECHANISM 177 



On the other hand, each organ of a special 

 sense has a terminal organ, such as the retina 

 in the eye, the various organs in the skin 

 connected with touch, and the wonderful 

 arrangements in the internal ear suitable 

 for being acted on by the vibrations of sound. 

 There are also end-organs in muscle and in 

 tendons by which nervous impulses are 

 awakened in these structures by movements, 

 and the impulses so generated are carried 

 to nerve centres. We do not know how end- 

 organs act. Those of muscle, for example, 

 may in some way excite the muscle protoplasm 

 so as to cause a kind of physiological explosion 

 ending in the inevitable contraction. We do 

 not know how the nervous impulse acts on 

 a secreting cell. Sensory end-organs, on the 

 other hand, as we shall see in considering 

 the senses, are each adapted to the re- 

 ception of their specific kind of stimulus. 

 Thus the retina is adapted to light, the 

 structures in the internal ear to sound, the 

 structures in muscle and tendon to pressure, 

 and so on 



99. We have next to turn our attention to 



H 



