SEEING 441 



it into the darkened space within the eyeball. The retina acts 

 as the sensitized plate in the camera, for on it are received the 

 impressions which are transformed and sent to the brain and result 

 in sensations of sight. The eye, like the camera, has a lens. This 

 lens is formed of transparent, elastic material. It is directly behind 

 the iris and is attached to the choroid coat by means of delicate 

 ligaments. In front of the lens is a small cavity filled with a 

 watery fluid, the aqueous humor, while behind it is the main cavity 

 of the eye, filled with a transparent, almost jelly-like, vitreous 

 humor. The lens itself is elastic. This circumstance permits a 

 change of form and, in consequence, a change of focus upon the 

 retina of the lens. By means of this change in form, or accommoda- 

 tion, we are able to see both near and distant objects. 



Practical Exercise 4. Make a diagram to show exactly what changes take 

 place in the eye when you look from your book out of the window to focus on 

 something coming down the far end of the street. 



Self-Testing Exercise 



There are areas on the skin that are sensitive to (1), 



(2), and (3). The organs of taste are the (4) 



(5). The kinds of substances distinguished by taste are 



(6), (7), (8), and (9). The 



sense of smell is located in the (10) lining the (11) 



part of the nose. The organ of hearing is the (12). It is 



composed of the (13), (14), and (15) 



(16). The (17) receives the sound waves which 



are transmitted to the (18) by the (19) (20). 



The eye is covered by three coats (21), (22), and 



(23). Impressions of seeing are made on the (24), 



which are carried to the (25) by the (26) 



(27). The (28) in form of the lens of eye is 



called (29). 



PROBLEM VI. WHAT BEHAVIORS ARE INSTINCTIVE? 



Demonstration 5. What actions of a newly hatched chick are in- 

 stinctive ? 



Place a newly hatched chick on a small tray, with food and water. 

 Place on the tray small, bright-colored, distasteful substances. Watch 

 the chick and make careful record of all its actions. List as many as 

 you can as instinctive. Are instinctive acts always useful? 



