338 THE VERTEBRATE BODY 



created by this expansion. This is known as inhalation or inspiration. 

 Exhalation, or expiration, occurs when the ribs are lowered and the 

 diaphragm is raised — these actions decrease the size of the chest cavity 

 and force air back out through the nose or mouth. This method func- 

 tions only so long as the chest cavity is intact — should it be punctured, 

 outside air could enter between the lungs and the chest wall and there 

 would be no lung expansion accompanying chest expansion. Strange to 

 say, the chest cavity is sometimes punctured purposely as a means of 

 treatment for tuberculosis. A needle is used to puncture the chest wall 

 and air is injected in between the chest wall and the lung. This causes 

 a collapse of the lung which gives it a maximum opportunity to heal 

 when it is thus freed of its duties in respiration. The chest cavity is in 

 two separate sections and the lung on the opposite side can go ahead and 

 function normally. 



The Circulatory System. Circulation is one of the most vital and 

 interesting of the life processes. It may seem dull if studied as an in- 

 animate group of vessels, but if we could visualize the activities of this 

 system by first-hand observation the wonders of the world would seem 

 dull in comparison. Suppose we make a tour of the circulatory system 

 of the frog as we would tour Carlsbad Caverns or some other great scenic 

 wonder. Of course, we are much too large, with our gross macroscopic 

 eyes, to begin to see the details of this system, so we must make ourselves 

 smaller, say about the size of a medium-sized bacterium, or one five- 

 thousandth of an inch in height. 



We start our journey on the foot of a frog which seems to stretch out 

 for miles on either side of us. We find an abrasion in the skin where 

 blood is oozing out, so we enter one of the capillaries through this open- 

 ing. We cling to the side of the capillary to get our bearings before 

 starting our journey. The capillary seems as wide as a river and the 

 blood is rushing past rapidly. We have read a guide book and are able 

 to identify the parts of the stream flowing past. The liquid part is a 

 clear, straw-colored fluid, which is the plasma. In it are floating huge 

 bodies of different sizes and shapes. Most of these are large, elliptical, 

 flattened bodies, which seem to be about twenty by thirty feet, and bear 

 an elliptical nucleus in their center. We notice that they are different 

 colors, some are bright red and others are a dull dark red. We see one 

 of the bright ones approaching us and we duck into a small cavity to 

 keep from being crushed. It bumps against the capillary wall and ad- 

 heres to it for a few moments. There, before our very eyes, we see the 

 bright red color fade into the dull red hue. As it pulls away, we remem- 



