MECHANICS OF THE RESPIRATORY MOVEMENTS 1089 



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 composed 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 hi contact with others on all sides. Imme- 

 diately below the squamous epithelium ramify blood- 

 capillaries derived from the pulmonary 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. 



FIG. 497. Diagrammatic re- 

 presentation of the struc- 

 ture of the lungs. The 

 trachea branches into two 

 bronchi, which subdivide 

 again and again before 

 ending hi the infundibula. 

 (From YEO.) 



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. 

 This is however much influenced by various conditions of the body, and 

 also by the age of the individual. Thus a newborn child breathes about 14 

 times a minute, a child of five about 26 times, a man of twenty-five about 16, 

 and of fifty about 18. The frequency is increased by any muscular effort , 

 so that even standing up increases the number of respirations. These 

 movements are much affected by psychical activity ; they are to a certain 

 extent under the control of the will, although they can occur in an animal 

 deprived of its brain, and are normally carried out without any special act 

 of volition. We can breathe fast or slow at pleasure, and can even cease 

 breathing for a time. It is impossible however to prolong this respiratory 

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