1040 PHYSIOLOGY 



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 composed of fibrous and elastic tissue and 

 a coating of unstriated muscular 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 com- 

 posed of connective tissue containing 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 pul- 

 monary artery. These form a close network, 

 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 surrounding 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 

 endothelial 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 movements of 

 the thorax, which constitute normal breathing. With inspiration the cavity 

 of the thorax is enlarged, and the lungs swell up to fill the increased space. 

 The capacity of the air-passages of the lungs being thus increased, air is 

 sucked in through the trachea. The movement of inspiration is followed 

 by that of expiration, which causes diminution of the capacity of the thorax 

 and expulsion of air. At the end of expiration there is normally a slight 

 pause. The number of respirations in the adult is about 17 or 18 a minute. 



FIG. 485. Diagrammatic re- 

 presentation of the struc- 

 ture of the lungs. The 

 trachea branches into two 

 bronchi, which subdivide 

 again and again before 

 ending in the infundibula. 

 (From YEO.) 



