86 



HUMAN PHYSIOLOGY 



two surfaces of the pleura and undue friction is thus 

 obviated. With regard to the structure of a lung it 

 may be described as a bag of air cells. If we follow 

 the track of the air breathed in we may readily be- 

 come acquainted, not merely with the actual desti- 

 nation of the air, namely the blood current, but also 

 gain at the same time a correct notion of the 

 structure of the lung. The windpipe divides at the 



larynx. 



aorta 



left lung heort right lung 



Fig. 25. THE RELATIONS OF HEART AND LUNGS. 



root of the neck into two main branches, each called 

 a bronchus (Figs. 24 and 26). The main division 

 passes into the lung of its own side and immediately 

 begins to divide and sub-divide into smaller and 

 smaller branches which are known as bronchial tubes 

 (Fig. 26). The windpipe and bronchial tubes are 

 lined by a delicate membrane which exhibits a special 

 feature of its own in the fact that its cells each 



