RESPIRATION 



387 



Rib Muscles. Second in importance are muscles between the 

 ribs which lift them up and outward, thus enlarging the lung cavity, 

 but, which is more important, bending the elastic rib cartilages, 

 which tend to spring the ribs back in place. 



Air Pressure. The third important factor in inspiration is the 

 pressure of the outside air 

 which rushes in to occupy 

 the extra space thus pro- 

 vided and by so doing, 

 expands the elastic tissue 

 of the lungs. Inspiration, 

 then, consists of (1) de- 

 pression of diaphragm and 

 compression of abdominal 

 organs, (2) raising the 

 ribs and bending the rib 

 cartilages, (3) air pressure, 

 expanding the lung tissue. 



Expiration. Expiration 

 is merely the springing 

 back of the organs that 

 have been compressed by 

 the movements of inspira- 

 tion. It consists of the 

 following steps: (1) the 

 elastic reaction of the com- 

 pressed abdominal organs, 

 (2) the springing back of 

 the rib cartilages, (3) the 



I 



FIG. 125. Lower half of thorax with 

 dorsal and lumbar vertebrae. A, sixth 

 dorsal vertebra; Ao, aorta; D, (lower) 

 diaphragm; D, (upper) aorta passing 

 through diaphragm; /, intercostal muscles; 

 O, cesophagus; IV, opening in diaphragm 

 for vena cava ascending; T T, tendons of 

 right and left crura attaching diaphragm 

 to 3rd and 4th lumbar vertebrae. (After 

 Allen Thomson.) From Kellogg. 



contraction of the elastic 

 lung tissue. 



All of these tend to make the lung capacity less and force out 

 the air, against its own pressure. The change of position of the ribs, 

 diaphragm and abdominal organs can be felt in our own bodies. 



Rate of Breathing. This double process takes place from 16 to 

 24 times per minute, depending upon activity, position, and age. 



