412 



THE NERVES AND THE SENSE ORGANS 



of the quality of food before we swallow it. What we call the ' ' taste " 

 of a substance is often a combination of smell and taste. Some of the 

 " strong" brands of cheese, for example, have a mild taste if we hold 

 our noses while eating them. 



412. Smell. The sense of smell, like the sense of 

 taste, is at the opening of the alimentary canal. But the 

 sense of smell guards our organs of 

 respiration as well as those of digestion, 

 for it passes upon the good or bad 

 quality of the air we breathe. Smell 

 can recognize only gaseous substances, 

 and not all of those. Thus, air, 

 oxygen, and hydrogen are odorless. 

 The neurons that send odor impulses 

 to the brain end at the olfactory cells ; 

 these are in the upper part of the nose 

 cavity (Fig. 298). 



Fig. 298. 



Sense of Smell; the 

 Olfactory Cells. 



Wild animals and primitive men have an 

 acute sense of smell, and can easily recognize 

 odors at some distance, but civilized man usually 

 has the sense in only a blunted form. For him 



all odors are classified as pleasant or unpleasant, and he does not 

 distinguish them further. The trained scientist often develops his 

 sense of smell to a high degree, so that he is able to recognize sub- 

 stances by their odors even when very small amounts of the 

 substances are present. 



413. Hearing; Structure of the Ear. We have already 

 learned that sound is due to the vibrations of some body 

 (cf. 189), and that the vibrating body sets up sound 

 waves in the air. The ear (Fig. 299) is the organ of 

 hearing; in it are the afferent neurons that respond to 



