They will swallow the bacteria and pass sand grains by. Our white cor- 

 puscles react to various kinds of bacteria much as the ameba reacts to 

 chemical stimuli (see page 188). In the retina of the eye light brings about 

 chemical changes, just as it does in chlorophyl or in a photographic film. 

 But in the retina, the chemical action sets up nerve stimulations. 



The tip of the tongue is more sensitive to touch than are the tips of the 

 fingers. Yet we think of the tongue not as a touching organ, but as a tast- 

 ing one — that is, an organ sensitive to chemical stimulations. Touch and 

 taste are related, however, since chemical action takes place only when two 

 substances come in contact. Another related sense is that by which we dis- 

 tinguish odors. In both tasting and smelling, stimulation depends upon the 

 presence of particular substances in direct contact with the nerve-endings. 

 These materials dissolve in water and diffuse directly into the sensitive 

 cells. 



The special receptors of taste are very small projections on the upper 

 surface of the tongue and in other parts of the mouth lining and of the 

 pharynx. These contain nerve endings connected with the brain cells, 

 through which we are made aware of taste. Our taste system can distin- 

 guish four classes of tastes: sweet, sour, salt, and bitter. 



The lining of the nose is sensitive to touch, as well as to odor. The 

 sneeze reflex is started by either a strong odor stimulation or by a touch on 

 some of the nerve endings in the nostrils. The sense of odor, in which the 

 chemical stimulant touches the surfaces in a volatile state, is much more 

 acute in many insects and lower mammals than it is in man. 



The little "stone" in the cavity of 

 the statocyst rolls about freely as 

 the body changes its position. 

 As it moves about in this way, it 

 comes in contact with delicate 

 hairs that line the cavity, now 

 touching one group, now another. 

 These hairs are outgrowths of 

 sensitive 'cells which connect with 

 nerve cells. These nerve cells in 

 turn are connected with muscles, 

 forming reflex arcs. As different 

 parts of the lining are stimu- 

 lated, different skeletal muscles 

 are made to contract. In this 

 way the animal retains or recov- 

 ers its position in relation to the 

 horizontal 



BALANCING ORGAN IN A SNAIL 

 288 



