TOUCH 215 



In addition to the senses of pain, touch, cold and heat, 

 the skin of some animals contains receptors for other senses. 

 There is a generalized chemical sense that in us is mainly 

 localized in the lining of the mouth and nose, which responds 

 to things that give sensations of astringency, pungency and 

 so on, that are neither tasted nor smelt. This sense is probably 

 of greater importance in the animals with naked and moist 

 skins, such as the amphibians and fishes, and many of the 

 invertebrates. 



In the sharks and rays among the fishes there is, too, an 

 elaborate arrangement of sensory pits in the skin over most 

 of the head. The pits are large enough to be plainly visible 

 to the naked eye after the removal of the skin, through which 

 they open by small and rather inconspicuous pores. These 

 "ampullae of Lorenzini" are filled with jelly and are well 

 supplied with nerve-endings. Experiments have shown that 

 they respond to changes in water-pressure, and it is probable 

 that they function as organs of "touch at a distance", for 

 changes in pressure will be produced when the fish move 

 near other objects, or when other moving objects are near, 

 or when wave movements of the water impinge on fixed 

 objects. The fish are thus able to feel things without touch- 

 ing them, through alterations in the water-pressure caused 

 in these diverse ways. 



A special development of the temperature sense is known 

 in a small group of animals — the ability to detect infra-red 

 radiation, the radiation of longer wave-length than that 

 producing the sensation of red light in our eyes, but of 

 shorter wave-length than that producing the sensation of 

 heat in our temperature receptors. This adaptation is found 

 in some of the snakes that live on small mammals such as 

 mice. They are the pit-vipers, which take their name from 

 the presence of a small pit in the skin on the side of the 

 head below and in front of the eye. The pit-vipers are 

 poisonous snakes found throughout the Americas, and all 

 the warmer parts of Asia ; they include the rattlesnakes, the 

 Copper-head or Moccasin, and the dreaded "Fer de lance". 

 The pit is plentifully supplied with nerve endings which 

 respond to stimulation by the infra-red radiation given off 

 by the warm bodies of small mammals and birds. The snakes 



