HUMAN PHYSIOLOGY 



THE AIR CELLS. Tracing one of the divisions of 

 the windpipe to its end, we find that it ultimately ex- 

 pands into a clump of little cells or compartments 

 (Fig. 26) varying in diameter from l-40th to l-70th of 

 an inch. The end of the bronchial tube in this way 

 may be compared to a passage from which opens a 

 series of little rooms arranged in a somewhat circular 

 fashion. These little rooms are the air cells of the 



lung. All a- 

 round the air 

 cells a network 

 of very fine 

 bloodvessels 

 exists. These 

 represent the 

 (Fig. 27) net- 

 work which a- 

 rises from the 

 division of the 

 bloodvessels 

 coming from 

 the right side of 

 the heart, whilst 

 they also re- 

 present the beginnings of the vessels returning pure 

 blood to the heart's left side. Each little clump .of 

 air cells is in fact a lung on its own account, so that 

 when air is breathed in and passes down the minute 

 sub-divisions of the windpipe it ultimately arrives at 

 the air cells of the lung, encompassed, as we have seen, 

 on every side, by bloodvessels bringing impure blood 

 from the body. The real work of the lung takes place 

 in the air cells, for according to a physical law, that 

 of "the diffusion of gases," the air breathed in 

 passes through the thin walls of the air cells and 



Fig. 27.THE DENSE NETWORK OF 

 BLOODVESSELS IN THE LUNGS. 



