RESPIRATION 199 



has made use of the first principle. Thus, the frog forces air 

 into its lungs by a swallowing movement. In artificial respira- 

 tion, 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. The pressure of 

 the air in the alveoli during the rapid expansion of the 

 lungs 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 

 expiration. 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 atmosphere, 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 and a 

 peripheral ring of muscular tissue, bulges up into the thorax 

 in the form of a flattened dome, and closes its lower 

 aperture. When the diaphragm contracts, the central 

 tendon descends ; the acute angle which the muscular ring 

 makes during relaxation with the thoracic wall opens out 

 around its whole circumference, so as to form a deep groove 

 of triangular section. The lungs follow the descending 

 diaphragm, their lower borders keeping accurately in contact 

 with it, while their apices move very slightly or not at all. 

 Since the diaphragm is attached to the lower ribs, there is a 



