BREATHING 281 



the nose. Finally, it becomes nearly saturated with 

 water vapor. We may breathe a cool, foggy air which 

 is already saturated and still impart moisture to it 

 for, as we raise its temperature, it gains in the capacity 

 to hold water. The three actions named filtration, 

 warming, and moistening are probably better per- 

 formed by the nose than they can be by the mouth. 



The purpose of the breathing movements is to renew 

 the air in the terminal sacs of the lungs. Before we 

 can make clear the means by which these movements 

 are executed certain mechanical conditions which pre- 

 vail in the thorax must be dealt with. First of all, the 

 fact is to be emphasized that the lungs, like other viscera, 

 are not directly attached to the chest wall. In dissect- 

 ing a normal animal we find no trace of adhesion between 

 the two pleural surfaces, the one lining the thorax and 

 the other making the exterior of the lungs. The only 

 connection between either lung and the rest of the body 

 is through its bronchus and the pulmonary vessels. 



Next we must note the fact that when an animal is 

 dissected the lungs appear much smaller than the cavity 

 which they completely filled in life. They have col- 

 lapsed and contracted to a half or a smaller fraction 

 of their former size. Sometimes one has a glimpse of 

 the lung in the act of falling away from the body wall 

 as the scalpel goes through it. After their collapse 

 they can be brought back to their normal size by blowing 

 through a tube which has been tied into the trachea, 

 but they will promptly shrink again if the air pressure 

 is discontinued. Yet the force needed to hold them 

 to their full capacity is surprisingly small. 



The collapse of the lungs when the thorax is opened 

 is proof that they were "always tending to do just this 

 thing always drawing inward upon the chest wall. 

 As has been explained in Chapter III, actual separation 

 between the organs and the body wall cannot occur 

 unless air or liquid is admitted between them. To part 

 them otherwise would require the immense force neces- 



