MECHANICS OF RESPIRATORY MOVEMENTS 1163 



below into two main branches bronchi ; and these subdivide again 

 and again, becoming gradually smaller. The terminal ramifications 

 or bronchioles open into rather wider parts the injundibula, the walls 

 of which are beset with a number of minute cavities, the alveoli. The 

 larger tubes are kept patent by rings or plates of cartilage in their 

 walls. The smaller tubes have no cartilage, their walls being com- 

 posed of fibrous and elastic tissue and a coating of unstriated mus- 

 cular fibres, which are able by their contraction to occlude the passage. 

 The whole system of tubes is lined with a layer of epithelium-- 

 ciliated columnar in the trachea, bronchi, and bronchioles, and 

 cubical over the parts of the infundibulum not occupied by air- 

 cells. The alveoli are the special respiratory parts of the lung. Their 

 walls are composed of connective tissue con- 

 taining a large number of elastic fibres, and 

 are covered internally by a single layer of 

 extremely thin large flattened cells. The 

 alveoli are closely packed together, so that 

 in a section of the lung an alveolus is seen 

 to be in contact with others on all sides. 

 Immediately below the squamous epithelium 

 ramify blood-capillaries derived from the 

 pulmonary artery. These form a close net- 

 work, and the blood in them is in proximity 

 to air on two sides, being separated from the 

 air in the alveoli only by the thin endothelial 

 cells of the capillary wall and the flattened 

 cells lining the alveoli. 



The lungs in their development grow out 

 from the fore part of the alimentary canal 

 into the front part of the body-cavity on 

 each side the pleural cavity. The sur- 

 rounding body- walls become strengthened by 

 the formation of the ribs, so that the lungs 

 are suspended in a bony cage-work, the thorax. Their outer surface is 

 covered with a special membrane, the pleura, which is reflected on to 

 the wall of the thorax from the roots of the lungs, and completely 

 lines the cavity in which they lie. The surface of the pleura facing 

 the pleural cavity is lined with a continuous layer of flattened endo- 

 thelial cells, and is kept moist by the secretion of lymph into the 

 cavity. Thus, being attached to the thorax only where the bronchi 

 and great vessels enter, the lungs are able to glide easily over the inner 

 surface of the thorax, with which under normal circumstances they 

 are in intimate contact. 



A constant renewal of the air in the lungs is secured by move- 



FIG. 486. Diagrammatic re- 

 presentation of the struc- 

 ture of the lungs. The 

 trachea branches into two 

 bronchi, which subdivide 

 again and again before end- 

 ing in the infundibula. 

 (From YEO.) 



