50 ELEMENTARY PHYSIOLOGY. 



skull and keep it in shape. The lower part of the 

 nose is shaped by a frame of cartilage, the advan- 

 tage of which over a nose-frame of solid bone you 

 can readily see. 



Things which have an odor, or smell, 



MM.O^W \W O 



Perceive give out little particles of matter, alto- 

 gether invisible. As these float in the 

 air, they are drawn into the breathing passages of 

 the nose and mouth at every breath. Of course 

 those which pass with the air into the mouth can 

 make no impression of smell, for there are no nerves 

 there which are affected by odors. But those which 

 pass into the nostrils strike upon the olfactory nerve 

 branches which, as we have seen, have their special 

 location there. The mind, receiving these impres- 

 sions, recognizes the odor, which may be feeble or 

 strong, agreeable or very unpleasant. 



The sense of smell affords us protection in two 

 important ways. Its organ, the nose, is set at the 

 very gates of entrance of the air we breathe and the 

 food we eat. So when the air is filled with putrid 

 or offensive invisible matter, which, of course, would 

 make it unfit to breathe,, we are cautioned by the 

 sense of smell, and instinctively turn away and seek 

 a purer air to breathe. Likewise, we are often 

 warned, just in time, against putting into the mouth, 

 as food, substances whose odor betrays their unfit- 

 ness to be eaten. Fortunately it is so provided in 

 nature that poisonous and other harmful substances 



