>f> PHYSIOLOGY FOR NURSKS 



and frame. Aii increase in the si/c of tlie thorax, there- 

 fore, makes the air pressure in the lungs less than the 

 pressure of the atmosphere and air enters the lungs un- 

 til the pressure is equalized. Forced inspiration means 

 simply forced increase in the size of the thorax requir- 

 ing a corresponding amount of air to establish equilib- 

 rium. The ribs slope downward and forward and are 

 fixed behind. Consequently, when the anterior end is 

 pulled upward they must push the sternum forward and 

 increase the diameter of the chest from before back- 

 wards. At the same time the spaces between them in- 



Fig. 9. Diagram of structure of lungs showing larynx, bronchi, bronchioles, 

 and alveoli. (Pearce-Macleod, Fundamentals of Unman Physiology.) 



crease and the chest grows from above downwards. If, 

 now, the diaphragm, which is fastened around the bar- 

 rel-like chest, convex upwards, contracts, it must flatten 

 the dome and greatly increase the space within the 

 thorax. It must be borne in mind that this space con- 

 tains not only the heart and lungs, but the great vessels, 

 and that the force which causes inrush of air can not but 

 produce some suction (aspiration) which will influence 

 their contents. As soon as the inspiratory muscles 

 cease to act, the abdominal contents, resting against the 

 under surface of the diaphragm, and held in place by 



