208 PHYSIOLOGY FOR DENTAL STUDENTS. 



bronchi. The lumen of the trachea and bronchi is maintained 

 patent by cartilage plates, which are imbedded in the walls of the 

 tubes. The bronchioles, however, have no such plates, their walls 

 being composed of fibrous and elastic tissue, in which is a layer 

 of smooth muscle. The whole system of tubes is lined with a 

 layer of ciliated epithelium. 



The bronchioles terminate in wide air sacs or cavities, tin- in- 

 fundibuli, from the walls of which extend numerous minute cavi- 

 ties, the alveoli. The walls of the alveoli are very thin but 

 strong, and are composed of a layer of elastic tissue lined with a 

 single layer of flattened epithelium. It is estimated that the epi- 

 thelial surfaces of the alveoli, if they were spread out on a flat 

 surface, would cover about 1,000 square feet. Such a large area 

 exposed to the air of the lungs offers the best of facilities for the 

 rapid exchange of the respiratory gases, and in fact the walls of 

 the alveoli are the true respiratory membrane of the lung, for 

 through them the exchange of gases between the air and the 

 blood takes place. Below the epithelial cells of the alveoli lie the 

 capillaries of the pulmonary artery in a regular meshwork; so 

 numerous, indeed, are they that each individual erythrocyte is 

 able to come in close contact with the air in the alveolus, separ- 

 ated only therefrom by the lining of the alveolus, the wall of the 

 artery, and the plasma of the blood. This arrangement makes 

 possible the rapid exchange of gases which must take place with- 

 in the lungs. 



The two lungs in company with the heart occupy the thoracic 

 cavity, which is bounded above and on the sides by the ribs and 

 their attached tissues, and below by the' diaphragm, a muscular 

 sheet of tissue which divides the body cavity into a thoracic and 

 an abdominal portion (Fig. 29). The lungs are suspended at 

 their roots, which are composed of the trachea and the pulmonary 

 blood vessels, and they lie free in the thoracic cavity in close ap- 

 position with the walls of the thorax. Covering the outside of 

 the lungs and the inside of the thoracic cavity, which is in con- 

 tact with the lungs, is a thin endothelial membrane known as the 

 pleura, the surface of which is kept moist by a secretion of 

 lymph. This smooth membrane allows the surface of the lungs 



