THE LUNGS AND RESPIRATION. 



205 



there is a gradual thinning out of the inner epithelium, 

 which in the final branches is reduced to a single layer 

 of ciliated cells. In the alveolus, finally, the wall is made 

 up of elastic tissue lined on the inside by a single layer of 

 flat epithelial cells. The very small bronchial tubes pos- 

 sess a little plain muscular tissue, so that they are actually 

 able to contract and expand, and thus reduce or increase 

 the amount of air which reaches the alveolus. The space 

 between the alveoli and the branches of the bronchial tubes 

 is filled with connective tissue, through which nerves, 

 arteries, and veins pass. 



Fig. 90. SECTION OF LUNG, SHOWING SEVERAL, CONTIGUOUS ALVEOLI, WITH THE BLOOD- 

 VESSELS IN THE SAME INJECTED. (After F. E. Schultze.) 

 a, a, c, c, partitions and edges of alveoli; 6, artery giving off capillaries. 



The lung is covered on the outside by a delicate serous 

 membrane, called the pleura. This surrounds the lung 

 closely at all points except at the root of the lung, the point 

 where the arteries and veins, and bronchial tubes enter it, 

 at which point the pleura is reflected and lines the inside of 

 the chest wall. 



