THE MECHANISM OF BREATHING. 



209 



to move easily over the inner surface of the thorax during the 

 changes in the size of the cavity which accompany the respiratory 

 movements. 



Mechanism of Breathing. Normal breathing has the object 

 of bringing about a constant renewal of air in the lungs, and it 

 is effected by movements of the thorax and diaphragm. When- 

 ever the cavity of the thorax is enlarged, as in the act of inspira- 

 tion, the lungs must increase in size to fill the space, and air is 



Fig. 29. The position of the lungs in the thorax. (T. Wingate Todd.) 



pushed into the respiratory tubules and the air sacs by the pres- 

 sure of the outside atmosphere. At expiration the reverse takes 

 place, and the air is expelled. A very good conception of the 

 mechanism by which this is brought about may be had by refer- 

 ence to Fig. 30. Any increase in size of the bottle, as by pulling 

 down the bottom rubber membrane, will cause air to expand the 

 rubber sacs coming in by the tube passing through the cork of the 

 bottle. When the size of the cavity is decreased by releasing the 

 membrane, the reverse takes place and air is expelled from the 

 rubber sacs. 



With every inspiration the thorax is increased in size, in all 



