THE AIR 



163 



t^^ 



to 



o 



o 





Lung cell 

 or air sac 







eral steps between the outside air and the Hving cells ; these are 

 pointed out in the diagram (Fig. 85). 



137. Control of breathing. When you wish to do so, you may 

 hold your breath for a minute or two, or modify the rate and 

 manner of breathing in other ways. Nevertheless our everyday 

 breathing is an 

 unconscious and 

 involuntary proc- 

 ess. The way you 

 breathe is a mat- 

 ter of habit; the 

 best control comes 

 from establishing 

 sound habits.^ We 

 need to practice 

 correct breathing 

 constantly rather 

 than take special 

 exercises to make 

 up for bad habits. 

 There are three 

 points in regard 

 to which faulty 

 breathing habits 

 are very common. 



I. Mouth breath- 

 ing. For several reasons we should acquire the habit of breath- 

 ing through the nose rather than through the mouth (see Fig. 86) . 



a. By means of the slimy mucus secreted by the lining of the 

 nostrils and by means of the hairs in the front part of the nos- 

 trils the dust particles are filtered out of the air we breathe. 



CO, 



Fig. 85. External and internal respiration 



The external respiration of the body consists of (i) the 

 muscular movements of the ribs and the diaphragm; 



(2) the air movements into and out of the lungs; and 



(3) the osmotic movements of the gases into and out of 

 the blood, through the linings of the air sacs. The inter- 

 nal respiration of the body cells consists of the exchange 



of gases between the cells and the blood or lymph 



^The rate of breathing is controlled automatically from certain nerve 

 centers, which are influenced by the proportion of carbon dioxid in the blood. 

 When you run fast, or exert yourself in some other way, the gas exchange in 

 the lungs (and so the gas exchange in the blood and in the working cells) is 

 increased; when you lie still, the gas exchange is reduced. 



