280 HUMAN PHYSIOLOGY 



A. Passage of air into and out of the Lungs. This is 

 brought about 



1st. By the movements of respiration breathing. 



2nd. By diffusion of gases. 



The air is made to pass into and out of the lungs by 

 alternate inspiration and expiration. 



I. Movements of Respiration A. Inspiration. During this 

 act the thoracic cavity is increased in all directions lateral, 

 antero-posterior, and vertical. As the thorax expands, the 

 air pressure inside the lungs keeps them pressed against the 

 chest wall, and the lungs expand with the chest. As_ a 

 .result of this expansion of the lungs the pressure inside 

 becomes less than the atmospheric pressure, and uir 

 rushes in until the pressure inside and outside airain 

 become e<mal. This can be shown by placing u lulu,- in 

 the mouth or in a nostril and connecting it with a water 

 manometer. 



This expansion of the lungs can readily be deter- 

 mined in the vertical direction by percussion, and in 

 the horizontal planes by measurement. By tapping 

 the chest with the finger over the lung in the right 

 intercostal spaces, a resonant note is produced, while if 

 the percussion is performed below the level of the 

 lung, a dull note is heard. If the lower edge of 

 this resonance be determined before an inspiration, 

 and again during it, it will be found to have 

 descended. 



As a result of inspiration, the form of the chest is 

 markedly modified, the change being best seen in trans- 

 verse sections. In expiration the chest in transverse section 

 is an elongated ellipse from side to side, in inspiration it 

 becomes more circular (Fig. 132). The change from side 

 to side and from behind forwards is best marked towards 

 the lower part of the chest, less marked in the upper part. 

 These changes may be recorded by means of a Cyrto- 

 meter, a piece of flexible gas tubing hinged behind, so that 

 it can be modelled to the chest. 



The change from above downwards cannot be directly 

 seen, but it is indicated by a forward movement of the wall 



