230 



BIOLOGY AND HUMAN LIFE 



(see c, Fig. 115). It is apparently through this organ also that 

 we become aware of falling or dropping, and that we manage 

 to keep our balance when walking, running, skating, etc. 



Everyone who wishes to become an air pilot must take a special kind 

 of examination for the purpose of discovering whether the balancing, or 

 equilibration, reflexes are in good working order. Unless a person can- 

 respond quickly to changes in bodily position he can never learn to con- 

 trol a machine that moves 

 in the three dimensions of 

 space, and often without 

 permitting the aid of 

 sight (see Fig. 116). 



189. Sight. Many 

 animals are very sen- 

 sitive to light without 

 having any eyes, and 

 many animals do not 

 distinguish light and 

 darkness. We know 

 that plants and the 

 ameba are sensitive 

 to light. These facts 

 mean that light is cap- 

 able of modifying the 

 processes that go on in 

 protoplasm. Only in 

 three of the main branches of the animal world is seeing possi- 

 ble ; these are the highest mollusks, the arthropods, and the 

 vertebrates. By seeing we mean not merely discriminating be- 

 tween light and dark but being able to distinguish forms and 

 colors at some distance from objects. 



Our own eye may be compared to a small camera with sensi- 

 tive nerve endings in the place where the film or plate would 

 be (see Fig. 117). The nerve endings in the retina of the eye 

 receive impressions from vibrations in the ether at the rate 

 of from 400,000,000,000 to 800,000,000,000 per second. If the 



Fig. 116. The three dimensions of space 



A solid body moves in a space which we think o*f as 

 extending in all directions. Every movement can be 

 thought of as a combination of movements in one 

 or more of the three planes representing the three 

 dimensions of space. The semicircular canals of 

 backboned animals are placed almost at exact right 

 angles to one another 



