BREATHING 285 



conies when the rising of the ribs more than compensates 

 for this. 



Expiration. Inspiration requires a marked exertion. 

 Expiration is promoted by certain factors other than 

 muscular contraction. Mention may be made of: (1) 

 the elastic tension of the lung tissue, lately referred to; 

 (2) the elastic reaction of the abdominal wall tending, 

 through the medium of the viscera, to thrust up the dia- 

 phragm; (3) the elastic reaction of the cartilages uniting 

 the ribs with the breast bone, and (4) gravity, for when 

 we breathe in we have usually to raise a weight. Muscles 

 are employed to a variable extent to compress the ab- 

 domen and draw down the ribs. The muscular activity 

 may be strenuous when expiration has a forced character, 

 as in shouting or blowing forcibly. 



The Closed Glottis. The glottis is the cleft between 

 the vocal cords in the larynx through which the breath 

 must pass. It is the narrowest segment of the respiratory 

 path. It can be very securely closed by the muscles 

 which flank it. When the glottis is closed the air in 

 the lungs becomes a substantial cushion. We in- 

 stinctively confine it when we make any great effort 

 and the upper part of. the body is thereby firmly sup- 

 ported. It is like screwing up the valve without which 

 the automobile tire has no power to bear weight. An 

 expiratory effort with closed glottis throws pressure 

 by means of the air cushion directly upon the heart, 

 hindering its diastole, and upon the veins, which may be 

 much compressed. The reddening of the face is a 

 sign of the backing up of the blood along the veins, and 

 the interference with the circulation may be great enough 

 to cause dizziness. 



The Tidal Air. When one is breathing quietly the 

 volume of the air which enters or leaves the nostrils at 

 a single breath is said to be about 30 cubic inches. This 

 quantity is called the tidal air because of its regular 

 coming and going. At each breath some air is carried 

 from the sacs of the lungs to the exterior and replaced 



