2,10 



A MANUAL OF PHYSIOLOGY. 



in, or they draw it in. In both cases the movement of the air 

 depends on the establishment of a slope of pressure from the 

 inlet to the interior. In the first method, this is done by 

 increasing the pressure at the inlet ; in the second, by diminishing 

 the pressure at the outlet. In certain animals Nature, in 

 solving its problem of ventilation, has made use of the first 

 principle. Thus, the frog forces air into its lungs by a swallowing 

 movement. In artificial respiration, as practised in physiological 



experiments, the same method 

 is usually employed : air is 

 driven into the lungs under 

 pressure. But in the vast 

 majority of air-breathing 

 animals, including man, the 

 opposite principle has been 

 adopted ; and the ' indraught ' 

 of air from nose and pharynx 

 to alveoli is not set up by in- 

 creasing the pressure in the 

 former, but by diminishing it 

 in the latter. This ' indraught,' 

 or inspiration, is brought 

 about by certain movements of 

 the chest-wall, which increase 

 tne capacity of the thoracic 

 elastic membrane tied on the bottle, cage and lower the pressure in 



FIG. 97. SCHEME TO ILLUSTRATE THE 

 MOVEMENTS OF THE LUNGS IN THE 

 CHEST. 



T is a bottle from which the bottom 

 has been removed ; D, a flexible and 



the thoracic cavity. The ex- 

 pansion of the highly-distensible 

 lungs keeps pace with the 



a glass tube fitted airtight through a 

 cork in the neck of the bottle. When 

 D is drawn down, the pressure of the 



and capable of being pulled out by 

 the string S so as to increase the 

 capacity of the bottle. L is a thin 

 elastic bag representing the lungs. It 

 communicates with the external air by diminution of pressure in the 



f\ rvlrt^ri 4-tiK^ -A + tj-tsJ ^Zf4-Z n-1-,.4- 4-U*-^,, ~V. ^ 



pleural sacs, and they follow at 



every point the retreating chest- 

 external air causes L to expand. When wall and diaphragm, although 

 the string is let go, L contracts again, ,1-1 11-11 



in virtue of its elasticity. the Y d not expand equally in all 



directions. The dorsal surface 



in contact with the vertebral column, the mediastinal surface in 

 contact with the pericardium and the contents of the mediastinum, 

 and the surface of the apex, move but little. The surfaces in 

 contact with the diaphragm, ribs, and sternum have the greatest 

 range of movement. Intermediate portions of the parenchyma of 

 the lungs expand in a degree determined by their distance from the 

 relatively stationary and mobile surfaces. The pressure of the 

 air in the alveoli during the rapid expansion of the lungs neces- 

 sarily sinks below that of the atmosphere, and air rushes in 

 through the trachea and bronchi till the difference is equalized. 

 Then commences the movement of expiration. The expanded 



