RESPIRATION 179 



As the air leaves the posterior nares, its course turns 

 sharply downward, and the stream impinges on the 

 posterior wall of the pharynx ; and it is here, perhaps, 

 more than anywhere else that particles of dust are most 

 likely to get entangled. It is not to be wondered at, 

 then, that the inhalation of dust is apt to produce a 

 pharyngitis. 



The functions of the nose in respiration, then, are 

 (1) to free the air to a large extent of suspended particles 

 and micro-organisms ; (2) to warm it ; (3) to saturate it 

 with moisture ; (4) to advertise us of the presence of 

 noxious gases, and, by a reflex mechanism, to prevent 

 the entrance of irritating fumes into the lungs. 



The disadvantages of mouth-breathing will now be 

 apparent, for the buccal cavity is not primarily designed 

 to fulfil the above requirements for the modification of 

 the air before its entry into the lungs. It is true that 

 the warming process is almost as efficiently carried out 

 by the mouth as by the nose,* but the absence of a 

 sufficient apparatus of secreting glands renders it almost 

 impossible for the mouth to moisten the air sufficiently 

 without its mucous membrane becoming dry and irritated 

 in the process ; hence the parched, cracked tongue 

 produced by the stertorous breathing of apoplexy and 

 the dry morning throat of the nocturnal mouth-breather. 

 Dust and micro-organisms, too, are less perfectly removed 

 by the mouth, and, reaching the bronchi, set up irrita- 

 tion there. This probably explains the frequent associa- 

 tion of bronchitis with adenoids and other causes of 

 nasal obstruction. 



* Kayser, Pfliiger's ArcUv, 1887, xli. 127. 



122 



