MECHANISM AND CHEMISTRY OF RESPIRATION 191 



in through the openings left for that purpose or a vacuum is 

 formed. The thoracic cavity is absolutely air tight; there- 

 fore, any motion which will enlarge the cavity of the chest 

 will draw air into the lungs. The chest is so constructed 

 that it can be enlarged in two different ways, (1) by rib mo- 

 tion and (2) by diaphragm motion. 



Rib Breathing. The ribs in the body are inclined down- 

 ward and forward; Fig. 101. They are hinged to the back- 

 bone and if their outer ends are lifted, the sternum will be 

 lifted and carried forward. This pushing forward of the 

 outer ends of the ribs increases the distance from the back- 

 bone to the sternum, and thus enlarges the cavity of the 

 thorax. The ribs are raised and lowered by two sets of 

 muscles, arranged between them, called the intercostals; 

 Fig. 101. When the external intercostals contract they lift up 

 the ribs, pushing forward the breastbone; when the internal 

 intercostals contract they pull the ribs down, thus drawing the 

 breastbone inward. But these same motions of the ribs cause 

 the chest cavity to enlarge and decrease in size laterally also. 

 This increase from side to side can be understood if we im- 

 agine a pail with two handles, one hanging down on either 

 side. If the handles are raised, each leaves the side of the 

 pail, and the distance between their outer curvatures is greater 

 than when hanging down. In precisely the same way, when 

 the external intercostal muscles raise the ribs the distance 

 between their curved sides is increased, for in their natural 

 position they lie inclined downward along the sides of the 

 body. When pulled back again to this position, the distance 

 between their sides is lessened and the thoracic cavity is made 

 smaller. 



From the method of their attachment to the vertebrae the 

 ribs can move only up and down. By looking at Fig. 123, 

 page 253, and imagining a rib attached to the centrum and then 

 tied by ligaments to the transverse process also, one sees that 

 the rib can move in but the one plane like a metal hinge joint. 



It is evident then that whenever the external intercostals 



