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 fundamental, 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 
RESPIRATION 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 func- 
tional parts. 
