MECHANISM OF RESPIRATION. 137 



particularly in the human subject, where they are usually 



more or less marked by the deposit of black pigment. The 



tissue between the lobules and that of the bronchial tubes 



require, like all other 



tissue, oxygenated blood 



for their nourishment ; 



and as that which ar- \_ 



rives by the pulmonary 



artery is unfit for use, ^ 



they receive their supply ^f^* 



from small systemic [ 



branches called the broil- 



chial arteries. 



Each lung is con 

 pletely invested with a ' 

 serous membrane, the 

 pleura,, the visceral layer 

 of which is adherent to 

 its surface, while the 

 parietal layer is reflected Fig. 77. LOBULES OP HUMAN LUNG, 

 from what is called the partially separated; natural size. 

 root of the lung, where the bronchus and blood-vessels enter, 

 to the walls of the chest, the outer surface of the peri- 

 cardium, and the upper surface of the diaphragm. 



99. The means by which the air is introduced into the lungs 

 and expelled therefrom, is similar to that by which it is 

 drawn into and forced out from, a concertina. The interior 

 of the lungs communicates freely with the air outside by 

 means of the windpipe, and when the capacity of the cavity 

 containing them is enlarged, the air passes in, while, when 

 the capacity is diminished, a quantity of air is expelled. 

 This is the principle of the mechanism, of respiration in the 

 majority of animals; but in frogs the want of osseous thoracic 

 walls, and in turtles their rigidity, renders it impracticable; 

 and therefore both these animals pump the air down into 

 their lungs by a motion of a swallowing description, which 

 may be seen constantly going on in their throats. 



100. The expansion and diminution of the chest in respira- 

 tion affects the whole thoracic cavity, containing the heart and 

 great blood-vessels as well as the lungs. It may, therefore, 



