CIRCULATION 227 



The air-tight thoracic cavity in which the heart and 

 lungs are situated is inclosed and protected by the ribs 

 and at the lower part by a dome-shaped muscle called the 

 diaphragm. Air enters the lungs because the muscles of 

 the chest pull the ribs so that they move upward and out- 

 ward and the muscles of the dome-shaped diaphragm cause 

 it to move downward. These two actions enlarge the 

 thoracic cavity. The air enters in the same way that it 

 enters a hollow rubber ball that has been compressed and 

 then set free. When the ribs move downward and the 

 diaphragm upward, the air is expelled as in the rubber ball 

 when compressed. 



There are then two ways in which air can be made to 

 enter the lungs, the " raising of the chest " and the move- 

 ment of the diaphragm. In the proper kind of breathing 

 these two movements go on together. The lungs are filled 

 throughout and not simply at either the top or bottom. 

 If this is to be accomplished, the body must be free 

 and not restricted by tight clothing about the chest or 

 the lower part of the trunk of the body, the abdomen. 

 Not only is the right kind of breathing necessary for 

 properly supplying the blood with oxygen, but also that 

 the lung tissues themselves may be properly nourished 

 and cared for. We should be particularly careful about 

 this now that infectious diseases of the lungs are so 

 prevalent. 



107. Circulation. Experiment 116. If a compound microscope 

 can be procured, tie a string tightly around the end of a clean finger, 

 and when it has become full of blood, prick it quickly with a sterilized 

 needle. Rub the drop of blood that comes out on a glass slide and 

 quickly examine under the microscope. Notice the great number of 

 round disk-like bodies, red corpuscles. Try to find an irregular-shaped 

 body which, while the blood remains fresh, slowly changes its shape, 

 a white corpuscle. These are rather difficult to find, but can be seen 

 if the drop of blood is thoroughly examined quickly enough. 



