CHAPTER VII 

 RESPIRATION 



THE expression respiration embraces two distinct ideas. It 

 may mean the entrance of oxygen into and the exit of carbon 

 dioxide from an animal, or it may have reference to the visceral 

 muscular and pulmonary, etc. movements by which these 

 gases are caused to flow in and out of the lungs. The lungs 

 of man are of vital importance in the interchange of oxygen 

 and carbon dioxide, while the skin is of but subsidiary impor- 

 tance. This condition of affairs is reversed in the frog. The 

 lungs consist of an enormous number of air vesicles or alveoli, 

 which communicate by means of a series of passages with the 

 trachea and the external air. Their total area is more than 

 one hundred times the superficial area of the skin, and their 

 walls form a delicate partition in intimate relation to the 

 blood capillaries of the lung. Before birth the lungs are airless 

 (atelectatic) , but after having once been expanded, they never 

 regain their atelectatic condition, because during collapse the 

 passages close first and so imprison some air in the alveoli. 

 This fact forms the basis of an important medicolegal test. 

 The lungs are enclosed in the air-tight thorax, and separated 

 from its walls by a double layer of pleura. The thorax of 

 the child grows faster than the lungs, so that the latter become 

 distended in an air-tight cavity. Whenever the thorax is 

 opened, the lungs, owing to the elasticity of their structure, 

 immediately shrink together. It follows, therefore, that the 

 lungs are always tending to shrink and thus pulling away from 

 the thoracic walls and diaphragm. This produces a pressure 

 in the pleural cavity below that of the atmosphere, and it is 

 called a negative pressure whenever atmospheric pressure is 

 regarded as a standard. The negative pressure has been found 

 to vary greatly under different conditions, but may be put 

 at minus 4.5 mm. Hg at the end of a quiet expiration, and 



