EESPIRATION 297 



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 air rushes 

 in until the pressure inside and outside again becomes equal. 

 This can be shown by placing a tube in the mouth or in a 

 nostril and connecting it with a water manometer. (Practical 

 Physiology.) 



This expansion of the lungs can readily be determined 

 in the antero-posterior direction by percussion, and in the 

 vertical 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 

 behind the level of the lung, a dull note is heard. If the 

 posterior edge of this resonance be determined before an 

 inspiration, and again during it, it will be found to have 

 passed backwards. (Practical Physiology.) 



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 ellipse from above downward, in inspiration it be- 

 comes more circular (fig. 136). The change from side 

 to side and from above downward is best marked towards 

 the hinder part of the chest, less marked in the anterior 

 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. (Practical Physiology.) 



The change from before backwards cannot be directly 

 seen, but it is indicated by an expansion of the wall of the 



