touch, to electrical disturbance, and to chemical action. Changes in tempera- 

 ture and light also stimulate protoplasm. In the more complex types of 

 animals, however, most of the protoplasm is inside the body and protected 

 against contact with happenings outside. Such animals receive stimuli 

 through special organs, just as they act upon their environments through 

 special effectors — hands and feet, for example, or jaws and teeth. 



Thousands of nerve endings in our skin are sensitive to slight pressure or 

 contact (see illustration, p. 217). The touch receptors are more closely 

 crowded in the tips of the fingers and on the tongue than in other regions. 

 There are also special end-organs sensitive to heat and others sensitive to 

 cold. The stimulation is carried along through one or more 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. The elevator 

 operator looks at the indicator and says, "Somebody rang on the tenth floor". 

 A button was pushed on the tenth floor, but he heard the bell wherever he 

 happened to be at the time, and he "knew" that the signal came from the 

 tenth floor because the indicator said "10" to him. 



Inside the organism, mechanical pressures or contacts may also act as 

 stimuli — the pressure of food in the intestine, for example, or the presence 

 of urine in the bladder. Some of these touch or pressure stimuli start re- 

 flexes; others bring impulses to the cortex and make us aware of the con- 

 dition or the position of the body. 



If you lie quietly with your eyes closed, you are still able to tell the posi- 

 tion of your body and of your limbs, because of nerve-endings which are 

 stimulated at the points in contact with the supporting surface. The vary- 

 ing tensions of the muscles attached to the bones of the skeleton give you a 

 feel of the relative position of the trunk and limbs. As you turn about, 

 changing strains of the floating viscera and variations in pressure on parts 

 that are not rigid contribute to the same feeling of position in space, or of 

 movement. In the inner ear is a special organ that seems to be directly re- 

 lated to the sensation of position-of-the-body and to sensations of spinning 

 and turning, which sometimes lead to dizziness (see illustration, p. 286). 



Our balancing organs are highly specialized contact receptors, which, 

 however, we do not ordinarily appreciate as we do our other sense organs. In 

 the swollen region near the end of each ear canal sensitive hairs project into 

 the liquid. When the head starts moving or turning, the liquid lags behind 

 somewhat, bending the hairs in the opposite direction. One "senses" the 

 changed position at this point. As the fluid's movement catches up with that 

 of the canal, the hairs become erect (see illustration, p. 287). 



In many crustaceans and molluscs there is a balancing organ, or statocyst, 

 which consists essentially of a hollow space with sensitive walls that con- 



285 



