n6 PHYSIOLOGY FOR BEGINNERS CHAP. 



Each infundibulum consists of a cluster of alveoli, into all of 

 which the fine bronchial tube conducts air. The lung is thus 

 made up of the closed dilated ends of an enormous number of 

 fine bronchial tubes, which are connected together by a little 

 fine connective tissue, the whole being covered by a trans- 

 parent elastic membrane, the visceral pleurS. In the body this 

 membrane, fragments of which may be stripped off the surface 

 of the lung, is continuous at the root of the lung with the 

 parietal pleura, lining that half of the cavity of the 'thorax. 



The walls of the alveoli consist of fine elastic connective 

 tissue, lined internally by a layer of flattened cells joined edge 

 to edge. Lying in the fine connective tissue, just underneath 

 the layer of flattened cells, is a close network of blood capillaries, 

 so that the capillaries are separated from the air in the alveoli 

 by this thin layer alone. The branches from the pulmonary 

 arteries run near the bronchial tubes and break up . into 

 the capillaries over the walls of the alveoli, the blood being 

 collected again by veins running back to the root of the lung, 

 and so to the heart. 



Tie a tube into the bronchus of the other lung and blow 

 air into it. The lung can thus readily be expanded to twice 

 its previous size, but when left to itself at once shrinks again, 

 driving the air out. By blowing air in, the alveoli are dilated 

 and their walls are put on the stretch, and when the blowing 

 in is stopped the elastic recoil of the walls drives the air out 

 again. Some air always remains in the lung ; if a bit of the 

 lung is thrown into water it floats because it contains air. 



The Natural Condition of the Lungs. When the lungs 

 are in the unopened thorax there is no air in the thorax round 

 the lungs, the air has no access to the outside of the lungs, and 

 the stiff walls of the thorax ward off the pressure of the 

 atmosphere from the outside of the lungs, hence the pressure 

 of the atmosphere, exerted through the air in the bronchial 

 tubes and alveoli, keeps the lungs distended so that each lung 

 fills up completely each lateral half of the thorax. The 

 pressure of the atmosphere keeps the lungs as much distended 

 with air as the size of the thorax will allow. But when the 

 thorax is opened, as by a wound in the chest, air enters 

 the thorax, and the pressure of the atmosphere is then 

 exerted on the outside also of the lungs, so that the pressure 



