MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 225 



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 increasing 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 the capacity of the 

 thoracic cage and lower the pressure in the 

 thoracic cavity. The expansion of the 

 highly-distensible lungs keeps pace with 

 the diminution of pressure in the pleural 

 sacs, and they follow at every point the 

 retreating chest - wall and diaphragm, 

 although they do not expand equally in 

 all directions. The dorsal surface in con- 

 tact 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 pres- 

 sure of the air in the alveoli during the 

 rapid expansion of theTungs necessarily 

 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 ex- 

 piration. The expanded chest falls back 

 to its original limits; the pressure in the 

 thoracic cavity increases; the distended 

 lungs, in virtue of their elasticity, shrink to their former volume; 

 the pressure of the air in the alveoli rises above that of the atmo- 

 sphere, and with this reversal of the slope of pressure air streams 

 out of the bronchi and trachea. 



In inspiration the chest dilates in all its diameters. Its vertical 

 diameter is increased by the contraction of the diaphragm, which, 

 composed of a central tendon, a peripheral ring of muscular tissue, 

 and the two muscular crura, bulges up into the thorax in the form 

 of two flattened domes, one on each side, and thus closes its lower 



15 



Fig. 109. Scheme to illus- 

 trate the Movements of the 

 Lungs in the Chest. T is 

 a bottle from which the 

 bottom has been removed ; 

 D, a flexible and elastic 

 membrane tied on the 

 bottle, and capable of being 

 pulled out by the string S 

 so as to increase the ca- 

 pacity of the bottle. L is 

 a thin elastic bag repre- 

 senting the lungs. It com- 

 municates with the external 

 air by a glass tube fitted 

 airtight through a cork hi 

 the neck of the bottle. 

 When D is drawn down, the 

 pressure of the external air 

 causes L to expand. When 

 the string is let go, L con- 

 tracts again, in virtue of 

 its elasticity. 



