37° 



THE ANIMALS AND MAN 



in solution. Tiiere are four primary taste sensations, namely 

 sweet, bitter, salty and acid, and these are distributed to 

 different parts of the tongue. All other tastes are com- 

 binations of these. A taste sensation is aroused by sub- 

 stances in solution only. 



Sense of odor. — The end organs of the sense of odor, or 

 the olfactory sensation, lie in the upper part of the nose, 

 just above the hard palate. They 

 are formed of groups of epithelial 

 cells each bearing on its free end 

 a little tuft of hairs. At the other 

 end the cell is continued into a 

 nerve fiber which passes back to the 

 olfactory bulb of the brain. During 

 inspiration air currents bring vapors 

 or gases into the nasal passages. 

 These are dissolved in the moisture 

 of the mucous membrane lining the 

 nasal passage and can then stimu- 

 late the hair-like processes of the 

 sensory cells. Substances that are 

 swallowed may send their vapors 

 into the nasal passage through the 

 opening of this passage into the 

 pharynx, and thus one sometimes 

 confuses the sense of taste and the 

 sense of odor. The stimulation 

 caused by the flavors of fruits, for instance, while really 

 affecting the olfactory organs seems to affect the taste organs. 

 Sense of sight. Structure of the eyes. — The eyes are 

 complex structures nearly spherical in form and set deeply 

 into the sockets of the cranium. Procure the eye of a sheep 

 or calf of the butcher, and study its parts. 



The eyeball is protected in front by movable folds of 

 skin called eyelids. The hairs along their edges serve to 



Fig. 184. Upper surface of 

 the tongue. 1, circumval- 

 late papillae; 2, fungi-form 

 papillae. (After Brubaker.) 



