138 RESPIRATION 



blood is furnished by the bronchial artery, which enters and 

 ramifies with the bronchi. The entire mass of venous blood 

 passes directly from the heart through the pulmonary artery to 

 the lungs to be arterialized, and it is the capillaries of this artery 

 which furnish the abundant network between the air cells. 



Pig. 50 Terminal branch of a bronchial tube, with its infundibula and air- 

 sacs, from the margin of the lung of a monkey, injected with quicksilver. 



a, terminal bronchial twig; b b, air-sacs, c c, infundibula. Xio. (Kirkes after E. E. 

 Schulze.) 



The lungs have the shape of irregular cones, their bases rest- 

 ing on the diaphragm and their apices extending to points a 

 little above the clavicles. They are completely separated from 

 each other by the mediastinum and their external surfaces are 

 covered by the pleura, a serous membrane similar to the peri- 

 toneum and reflected from the thoracic wall. The right lung is 

 divided by fissures into three lobes and the left into two. Super- 

 ficially the lung substance is seen to be subdivided into areas 

 about J in. in diameter called the lobules. Each lobule is com- 

 posed of a number of lobulettes as above mentioned. 



MECHANISM OF RESPIRATION. 



Respiration implies the more or less regular entrance and exit 

 of air to and from the lungs. The entrance is inspiration; 

 the exit expiration. Now, the thorax is a closed cavity, not- 



