THE SPECIAL SENSE ORGANS 227 



the very nature of life there could be no such perfect instru- 

 ments. Consider, for example, the fish. If the reflexes of the 

 fish were perfect, it would always get every bit of prey for 

 which it made a dash ; and if the reflexes of the other animals 

 were perfect, they would always escape their enemies. This, 

 you can see, is a contradiction or impossible condition. Never- 

 theless we may say that most reflexes are useful to the organism. 

 In the lowest organisms the reactions to stimuli are of few 

 kinds, and there is very Httle distinction between the effect of 

 one kind of stimulus and the effect of another. Thus, an ameba 

 may contract when touched, or when suddenty illuminated, or 

 when stimulated by some chemical or by a charge of electricity. 

 In our own bodies the division of labor has gone so far that we 

 have several highly specialized organs, each of which is sensitive 

 to only a limited class of stimuli. 



184. The skin. In the skin are delicate nerve endings that 

 are sensitive to slight pressure or contact (see Fig. 99). In 

 some parts of the body the touch organs are very close together^ 

 as on the tips of the fingers and on the tongue. It seems that 

 we perceive heat through the stimulation of certain end organs 

 in the skin, and cold through certain others. 



The disturbance, or stimulation, is carried along through the 

 neuron and is passed on through one or more other neurons 

 until it finally sets up a disturbance in one or more cells of the 

 brain cortex. Here the stimulus is at last translated into a 

 feeling, or sensation. We say that the finger is hot, but it is in 

 the brain that we feel the stimulus. 



185. Chemical sensitiveness. The simplest animals, like the 

 roots of many plants, are sensitive to many kinds of chemical 

 disturbance. We cannot suppose that the ameba, for example, 

 has the feeling of sour or sweet, or that the Paramecium has an 

 idea of nice or nasty. Yet it is very plain that the protozoa are 

 attracted by the presence of various kinds of bacteria, and that 

 they are repulsed by various chemical substances. They will 

 swallow the bacteria and pass sand grains by. We cannot say 

 the ''ameba likes meat juice" as we cannot say "water dislikes 



