98 PHYSIOLOGY. 



tary occupations, such as that of the student, not calling 

 for deep breathing (and often the air taken in is of poor 

 quality), need to pay especial attention to the matter. 



Breathing through the Mouth. We should breathe 

 through the nose, and not through the mouth. The nasal 

 passages are fitted for the introduction of the air (i) by 

 being narrow, but of large area; (2) by having their lining 

 membranes richly supplied with blood ; (3) by the abun- 

 dant secretion of mucus by this membrane. The air, 

 coming through this narrow channel, is warmed, and a 

 large part of any dust it may contain is caught by the 

 sticky mucus that covers all the walls of this passageway. 

 If we breathe through the mouth (especially out of doors 

 in cold weather), the air may not be sufficiently warmed 

 before entering the lungs, and much more dust would be 

 carried into the lungs. Then, too, the air has a drying 

 effect on the throat, whereas the mucus of the nasal pas- 

 sages will moisten the air as it enters. The cilia, which 

 extend from most of the cells lining the respiratory pas- 

 sages, are constantly causing the mucus to slowly flow 

 toward the external opening, so a good share of the dust 

 is gotten rid of. A further advantage of breathing through 

 the nose is that we detect odors, and can thus judge of the 

 quality of the air. 



Breathing and Circulation. The fact has been noted 

 that breathing directly aids the circulation of the blood. 

 This is due to the way air pressure is made to affect the 

 large veins. Breathing also may very considerably aid 

 the flow of lymph. Every deep inspiration brings pres- 

 sure to bear on the main lymph duct as the diaphragm 

 descends. Every forced expiration has the same effect. 

 We must keep in mind that the tissues are fed directly by 



