XVIII. 



THE RESPIRATORY TRACT. 



THE function of the respiratory tract is the exchange of gases, 

 mainly carbonic acid gas for oxygen, and for this purpose 

 there is free access of the atmospheric air to the lungs by means 

 of tubular formations. The nasal cavities must be considered as 

 the beginning of the respiratory tract, although the oral cavity 

 can serve for the same purpose. While all cavities and canals 

 of the body, when at rest, are closed by folds of the mucosa, the 

 cavities and passages engaged in the respiratory function are 

 open either by means of the surrounding bones and cartilages 

 such as the nasal cavities, or by cartilages alone, as the larynx, 

 trachea, and the bronchi. The lungs themselves, where the 

 gaseous exchange takes place in all higher developed animals, are 

 kept in open communication with the outer world by the elastic 

 tissue, which is present in large amount in the walls of the 

 alveoli. The lung contains air, " residual air," from the first 

 inspiratory movement of the newly born child, the amount being 

 merely augmented by inspiration and lessened by expiration. 

 In the foetus, before birth the alveoli of the lungs are closed, and 

 such lungs are called atelectatic. The same condition is pro- 

 duced by the choking of the alveoli by the products of inflamma- 

 tion, or a transformation of the lung-tissue into solid connective 

 tissue, or by the pressure of exudate in the pleural cavity. The 

 capillaries, like all structures of the lungs, are developed before 

 birth, but become filled with blood only after the first inspiratory 

 movement. 



(1) The two sides of the nasal cavity are divided into a respite- 



