304 PHYSIOLOGY. 



Why we Sniff. In quiet breathing the air passes 

 along the lower air passages just above the hard palate. 

 The true olfactory passages are higher, but still in com- 

 munication with this lower passage. When we wish to 

 test the quality of the air, we sniff, that is, make a sudden 

 inspiration by jerking the diaphragm down, and air from 

 the outside then rushes into these upper nasal passages, 

 over the walls of which the nerves of smell, the olfactory 

 nerves, are spread in the mucous membrane. The sudden 

 rush of air against this membrane seems to aid greatly in 

 detecting the odor. The nerves have peculiar endings, 

 and it is not known just how the substances produce their 

 effect. The substances must be in a very finely divided 

 state, probably gaseous. The mucous membrane is sup- 

 plied with mucus, and the odorous substance, probably, is 

 first dissolved in the mucus. The lower, or respiratory, 

 passages have a more abundant blood supply, and are 

 redder than the upper. In inflammation, owing to their 

 narrowness, the passages, especially the upper, are often 

 closed by contact of the opposite sides. Substances like 

 ammonia have no odor, but excite the tactile nerves. They 

 are often spoken of as having a " pungent " odor, but are 

 simply irritants. 



The Sense of Hearing. The ear passages are inclosed 

 by the hard bones of the head. The ear is, in consequence, 

 difficult to dissect. It is very desirable to have a model 

 of the ear. The ear may be dissected in a cat or rabbit by 

 following the accompanying description. It will take time 

 and patience to trace all the parts. 



The Parts of the Ear. The ear is a much more com- 

 plicated organ than would naturally be supposed. The 

 parts of the ear are the external, the middle, and the in- 

 ternal ear. 



