Chap. 17 RESPONSIVENESS THE SENSE ORGANS 311 



a cat's ear, and the vibrissae or whiskers of cats and rats, and less evidently 

 human hairs. 



In the insects and other arthropods, bristles are connected with receptors. 

 Although ants are armored in chitin they are exquisitely in touch with their 

 surroundings by way of their bristles. Human skin contains several types 

 of sense organs. By testing a small area of skin, point by point, receptors 

 for touch, pain, cold, and heat can be found at different locations. Pain may 

 be slight or very intense. The mild pain of pinpricks can be definitely located, 

 but pains deeper in the body are rarely so precisely determined. 



Temperature — Thermoreceptors 



Little is known about temperature receptors except those in higher animals 

 (Fig. 17.1). Protoplasm is sensitive to temperature although no receptors 

 may be present. Insects are highly responsive to it; ants move their young 

 from one to another part of their underground nests as the temperature 

 changes during the day. In temperatures around 30° C. and under controlled 

 conditions of humidity adult mosquitoes {Culex fatigans) react to differences 

 as slight as five-hundredths of one degree. 



Internal Senses of Muscles and Viscera 



Many impulses from these receptors reach the higher centers of the brain 

 and consciousness, but many others end in the spinal cord and cerebellum 

 below the level of consciousness. Proprioceptors are sensitive to changes in 

 the tension of muscle and tendon. Such changes stimulate impulses to the 

 brain, making us aware of the position and movements of our arms and legs 

 and other parts of the body, the interplay through which the body is kept 

 in a balanced position. Interoceptors are important in regulating the activities 

 of the lungs, alimentary canal, and other viscera in which they are located. 

 They bring about reflex control of internal organs through centers in the 

 medulla and thalamus of the brain. Some of these impulses go through to 

 the higher centers of the brain and are responsible for such sensations as 

 having had "a good dinner." 



Chemical Senses 



These are the common chemical sense and the twin senses of taste and 

 smell. All chemical receptors are alike in their requirement that particles of 

 a substance must be dissolved and in actual contact with them. We can taste 

 sugar only when it is chemically associated with certain taste buds; we can 

 smell roses, skunks, cheese, or vanilla only when their essences enter the 

 olfactory cells. 



Common Chemical Sense. The surface of the bodies of fishes and am- 

 phibians is sensitive to chemical substances of a very mildly stimulating 



