THE SPECIAL SENSES 399 



sensitive to each of them. The bitter taste is most 

 developed at the back of the tongue, the sour at the 

 sides, the sweet at the tip, and the salt taste is nearly 

 equally distributed. Many of 

 our tastes are combinations 

 of these four primary ones 

 together with smell. 



Smell. The sense of taste 

 and smell at the beginning of 

 the digestive tract and the 

 sense of smell at the begin- 

 ning of the respiratory tract 

 also will at once suggest a TiB ' 180 ' The tonsue ' 

 possible function for these two senses. They are un- 

 doubtedly of some use in helping us to detect unfit food 

 and air. 



The nerves of smell, the olfactory nerves, have their 

 endings in the mucous membrane of the upper part of 

 the nasal cavity. Substances which stimulate these 

 nerves give off small particles which are then carried 

 into the nose by currents of air. The sense of smell 

 is not so keen in man as it is in dogs, or in some other 

 lower animals. Even in man it is very acute, but it 

 becomes exhausted rather quickly. 



Eye. The eyes are delicate, complex, nearly spherical 

 bodies which fit into sockets formed by the facial 

 bones. They are further protected by folds of skin, 

 the eyelids. They are freed of dust and kept moist 

 by mucus and by tears secreted by the tear glands. 



