364 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



chest. Why this must be so is shown in diagram, Fig. 

 246. If 1, 2 and 3, 4 be two ribs connected with the 

 backbone behind and the breastbone in front, and there- 

 fore compelled to move together and in the natural slop- 

 ing position, and if a a represent one fiber of the exter- 

 nal sheet, it is evident that if this fiber contracts it will 

 elevate both ribs, because, although it pulls one up and 

 the other down, yet the leverage on 3, 4, by which it 

 pulls up, is greater than that on 1, 2, by which it pulls 

 down ; and since they must move together, the pair will 

 move up. But upward movement expands the chest. Or, 

 put it another way: Any position of the ribs will be 

 assumed which shortens the fiber a a to a! a! . For the 

 same reason contraction of the fibers of the interior 

 sheet (b' b') will depress the ribs and contract the chest, 

 because here the downward pulling acts with the greater 

 leverage on 1, 2'. Therefore thoracic or costal respira- 

 tion is accomplished by alternate contraction of the ex- 

 terior and interior intercostal muscles. The one pro- 

 duces /aspiration, the other expiration. 



Diaphragmatic or Abdominal Respiration. — This 

 is accomplished by the alternate contraction of the dia- 

 phragm and the abdominal muscles. When the muscular 

 fibers of the diaphragm contract, the high arch is brought 

 down and the dome flattens. This increases the vertical 

 diameter of the thorax as the intercostal contraction 

 does the transverse diameter. But the descent of the dia- 

 phragm presses on the stomach and liver, and these again 

 on the intestines on which they rest; and these in turn 

 press outward on the abdominal wall, which therefore pro- 

 trudes. This is Aspiration. The stretched abdominal 

 muscles reacting, by contraction, press on the intestines, 

 and these upward on the stomach and liver, and these 

 in turn lift the dome of the diaphragm and press on the 

 lungs, squeezing out the air. This is expiration. 



