504 APPLIED BIOLOGY 



428. Breathing Movements. Expansion of the chest- 

 cavity will result in reduced pressure so that the external 

 air will enter through the trachea until the pressure in the 

 air-chambers of the lungs balances that between the lungs 

 and walls of the chest-cavity. This is exactly what happens 

 in the following experiment. 



(D) Action of Diaphragm. Use apparatus constructed as follows : 

 A sheet of dental rubber is stretched and tied over the base of an 

 open-top bell-jar, or lamp-chimney that is wide at base and narrow 

 at top. A short glass tube is tied into the mouth of a delicate rubber 

 bag (e.g., toy balloon), and the free end of this tube is then inserted 

 through a hole in a cork which will fit tightly into the top of the 

 bell-jar. When the cork is placed, the rubber bag should hang near 

 the center of the jar. The glass jar represents the walls of the chest- 

 cavity, the glass tube represents trachea, the rubber bag stands for 

 elastic lungs, and the rubber sheet at the bottom acts as a dia- 

 phragm. Note that when this is made convex by pressure, the rub- 

 ber bag (imitating lungs) collapses. Why ? Why is air forced out of 

 any toy balloon when free to escape ? When the rubber diaphragm 

 becomes flat (imitating the downward or posterior movement of the 

 human diaphragm), the rubber "lungs" expand. Why? Why 

 does water rush into a pump when the piston is raised ? This ap- 

 paratus illustrates the mode of respiratory action ; but is not exact, 

 for the lungs fit and fill the chest-cavity, except that the heart lies in 

 a space between them. 



The expansion of the human chest-cavity is due to breathing 

 movements, which are of two kinds : (1) those of the dia- 

 phragm, whose positions, due to muscular movements, may 

 be imitated by the rubber sheet across the mouth of the 

 jar used in the preceding demonstration ; and (2) to expansion 

 of the side-walls, increasing the diameter in a horizontal 

 plane. This latter is due to raising the ribs, and can be 

 demonstrated by measuring with a tape the circumference 

 of the chest before and after inspiration. Young children 

 expand the chest-cavity chiefly by the movements of the 

 diaphragm; in many men the diaphragm does most of 

 the breathing work ; in some women, particularly when tight 



