RESPIRATION. 37 



This function is performed in the lungs, and the transfer is 

 effected by the agency of the blood. 



Describe the course of the air in entering the lungs. 



Air is taken through the nose or mouth, and passes through the 

 pharynx to the larynx ; entering the rima glottldis. it passes through 

 the larynx to the trachea and bronchi. The air is somewhat warmed 

 and moistened, in its passage. The trachea and bronchi are lined 

 with ciliated epithelium, which serves to sweep particles of dust 

 and the like out of the air-passages. 



Describe roughly the minute anatomy of the lung. x 



Each lobe of the lung is composed of numerous lobules, to each 

 of which a small bronchiole enters, and the minute terminal branches 

 of these bronchioles ^infundibula) widen into a sort of irregular 

 funnel having pouched or sacculated dilatations known as air- 

 cells. These air-cells are supported by numerous elastic fibres, and 

 are lined with a very thin layer of flat (not ciliated) epithelium. 

 Outside the epithelial lining is a very close mesh of capillaries, 

 which are often exposed to air on both sides by lying in a partition 

 between two of the air-cells. The air-cells or vesicles are about -fa 

 in. in diameter, and the space between the capillaries is often less 

 than the diameter of a capillary. 



What is the blood-supply of the lungs ? 



There are two sources : (1) from the bronchial arteries (systemic) 

 for nutrition ; (2) from pulmonary artery for oxidation. 



How is air introduced to the lungs? 



The thoracic cavity in which the lungs lie being a closed cavity, , . 

 the expansions of its walls will tend to create a partial vacuum ^ 

 in its interior. It is by this means that air is introduced. This 

 expansion of the chest-wall (and with it the lung) we call inspira- 

 tion. Contraction of the cavity of the thorax tends to cause com- 

 pression of its contents, and thus to expel the contained air : this 

 we call expiration. The complete act is called respiration. 



Describe briefly the mechanism of respiration. 



A. Inspiration. The diameters of the thoracic cavity are all in- // 

 creased (1) the vertical by the action of the diaphragm ; (2) the 

 lateral and antero-posterior diameters by the elevation of the ribs. 

 Thus we may have a diaphragmatic or abdominal type of breath- 



