The Respiratory System. 85 



through special perforations of the anterior portion of the head 

 and through the oral cavity. The principal connection in a 

 mammal is represented by an extensive nasal cavity bearing on its 

 lateral walls the olfactory sense-organs. It is distinguished as an 

 accessory respiratory tract from the true respiratory tract 

 formed by the trachea and its terminal divisions, the bronchi. 

 The respiratory system as represented by the lungs and related 

 tubes, is nominally ventral to the oesophagus, but this relation 

 is chiefly true of the trachea. In the thorax (Plate VII) the 

 bronchi are, in general, interposed between the oesophagus and 

 the heart, the lungs being expanded laterally into the paired 

 pleural cavities. 



In addition to carrying air over the sensory, olfactory surfaces, 

 the respiratory system has accessory functions in relation to 

 respiration. The mucous membrane of the nose, including that of 

 the turbinated surfaces, serves both to warm the air, and to remove 

 particles of foreign material. The chief function is, however, 

 respiratory. 



In a mammal, respiration is both a physicochemical and a 

 mechanical process. The former is fimdamental, and consists in the 

 supply of oxygen to the blood, and in this way to the tissues, for 

 the oxidative phases of metabolism; also in the discharge of waste 

 RSSPIRATION AS gases, principally carbon dioxide, from the 



A PROCESS. blood to the air. The absorption and 



transport of oxygen is a specific function 

 of the red blood cells. Though the oxygen, of which a certain 

 amount always remains in the lungs during the process of breathing, 

 must pass through the thin epithelial covering of the terminal air 

 sacs into the capillaries before it can be taken into the blood cells, 

 the latter from their flattened shape and very great numbers 

 present a relatively enormous surface for absorption, the process 

 being thereby facilitated. The lungs themselves are highly elastic, 

 expansible sacs. They have the structure of greatly ramified 

 saccular glands, except that the free internal surfaces are every- 

 where in contact with air. The division of the trachea into its 

 bronchi, together with the bronchial ramifications, are the trunk 

 and main branch portions of a rather complex system of tubes 

 (Fig. 45); of which the terminal air-spaces are the final and Junc- 

 tional parts. 



