RESPIRATION. 337 



THE RESPIRATORY APPARATUS. 



The object of respiration is twofold, viz. : to supply the oxygen 

 necessary for the numerous oxidation processes that are constantly 

 occurring within the body, as well as to remove the carbon dioxide 

 formed within the body. The most important organs for this pur- 

 pose are the lungs or the gills, as the case may be, though it must 

 never be entertained for a moment that they are the special seats for 

 those combustion-processes whereby carbonic acid ensues as the final 

 result. These processes occur in all parts of the body in the sub- 

 stance of the tissues. The lungs or the gills are merely the medium 

 for the exchange of the two essential gases. For this interchange it 

 becomes necessary that the atmospheric air should pass into them and 

 that the changed air should be expelled from them. 



In essence a lung or a gill is constructed of a thin membrane, 

 whose one surface is exposed to the air or water, depending upon 

 the species of animal, while on the other surface there is a network 

 of blood-vessels, the separating membrane between the blood and 

 aerating medium being the thin walls of the small blood-vessels and 

 the fine membrane upon which they are distributed. The principle 

 is always the same in all respiratory apparatuses; the difference 

 between the simplest and most complicated ones is one of degree 

 mly. 



In all animals in which, by reason of their complex structure, it 

 becomes necessary to have special arrangements for the performance 

 of the respiratory function it is found that the act is divided into 



stages: (a) an external respiration, where the interchange is 

 between the air or water on the one hand, and the circulating 

 medium of blood on the other, as it passes through richly vascu- 

 ir skin, tracheae, gills or lungs; (&) an internal respiration, which 

 an interchange between the blood or the lymph and the cells of the 

 various tissues of the entire body. 



Our consideration of the subject will confine us to the study of 

 the human respiratory organs. The most important of the human 

 ipparatus are the lungs, which are contained within the closed chest, 

 thorax, and have no communication with the outside except 

 irough the avenue of the respiratory passages. 



The pulmonary apparatus consists of: (1) the air-passages 

 i, pharynx, larynx, trachea, and the bronchi, which communicate 

 dth the lungs; (2) the lungs with their immense number of small 

 ics, known as the air- vesicles ; and (3) the thorax. The accessory 



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