2 8o Why Liv'mg 



receptors. There are taste buds on the 

 tongue and special nerve endings for 

 smelhng in the nose. Scattered in the skin 

 are tiny structures which receive stimuh 

 of pressure, touch, and cold; separate 

 ones receive each of these stimuli. There 

 are also bare nerve endings which when 

 stimulated by a variety of stimuli give 

 rise to pain. You can learn much about 

 these receptors by doing Exercises 6, 7, 

 8, 9, and lo. The skin is not equally sen- 

 sitive in all parts of the body; the recep- 

 tors and nerve endings are more concen- 

 trated in certain parts. For example, the 

 skin is more sensitive to touch and to 

 pressure on the finger tips than on the 

 back. The lips are more sensitive to pain 

 than are the palms of the hand. 



Up to this point you have thought of 

 receptors as being structures near the 

 surface of the body which bring your 

 body into relation with the environment. 

 But there are receptors inside the body, 

 too. For instance, there are receptors in 

 the stomach wall. When the empty stom- 

 ach contracts these are stimulated. 

 Through them you get the sensation of 

 hunger. Nerve endings, perhaps in the 

 throat or other parts of the body, when 

 they lack sufficient water, are responsible 

 for the sensation of thirst. There also are 

 many other internal receptors. All recep- 

 tors are important because they help 

 keep the body as a whole informed of 

 the internal and external environment. 



The eye. We must study the eye so 

 that we may know how to care for it. 

 The eyeball is almost spherical with a 

 lens suspended in it near the front. It is 

 a very complicated organ which has 

 highly sensitive receptors which are 

 stimulated by light. These receptors lie 



Things Behave As They Do unit v 



Liquid 



erve 



Cornea 



Lens 



Retina 



Fig. 266 A sbnplified diagra??/ showing the parts 

 of the eyeball. What are the three coats? 

 Through what must a ray of light pass before 

 it reaches the retina? 



in the ret'ma (ret'i-na), the layer which 

 lines most of the eyeball. The recep- 

 tors in the retina connect with the nerve 

 to the brain. Outside the retina is the sec- 

 ond coat (choroid). It is complete except 

 for a small opening directly in front of 

 the lens. This opening is the pitpil. A 

 small portion of the choroid coat can be 

 seen as the colored part of the eve (iris). 

 By changes in size this portion regulates 

 the size of the pupil, admitting more or 

 less light to the lens. The third coat 

 (the sclera) is tougher than the other 

 two. It completely surrounds the eyeball. 

 The part that covers the iris and lens is 

 transparent. It is called the cornea. 



Light rays pass through the transpar- 

 ent covering, then through the pupil, 

 and thus strike the lens. The lens directs 

 the rays to the retina. Thus an image is 

 formed on the sensitive cells of the ret- 

 ina. The lens becomes flatter or more 

 rounded in formino- an imaoe on the 

 retina. Figure 267 shows how focusing is 



