252 



HITCHCOCK'S ANATOMY 



the interior of the lungs. This apparent contradiction of 

 terms arises from considering that portion of the body as ex- 

 ternal which is in contact with the air, in the same manner as 

 the intestines ; the anatomically external surface being phys- 

 iologically the internal surface. 



444. The 8 of) stance of the Icings.- The substance of the 

 lungs is entirely made up of arteries, veins, lymphatics, and 

 bronchial tubes, connected by areolar tissue. And when in- 

 flammation takes place in this substance, the disease is known 

 as pneumonia, or lung fever. 



445. The Essential Muscle of Respiration — Process of 

 Breathing—The Rest Gained by the Respiratory Muscles 

 — The muscle which performs the most essential part of 

 breathing is the diaphragm. This by its contraction produces 

 inspiration, but does not directly produce expiration. Breath- 

 ing is performed as follows. The cavity of the chest is va- 

 riable in size ; made so by the movement of the diaphragm 

 Upwards and downwards. For as the sides and upper portion 

 of the chest are unyielding walls, if the diaphragm be de- 

 pressed the cavity of the chest is enlarged, a partial vacuum 

 is produced, and the air rushes in to supply the vacancy. 

 This is inspiration, and is purely an active muscular effort. 

 Expiration, how r ever, is quite the reverse, being wholly a pas- 

 sive exercise. For when the lungs are filled, the contents of 

 the abdomen are depressed by the descending diaphragm and 

 the ribs are elevated, both of which parts offer resistance to 

 the inhaling force. Consequently, when the diaphragm and 

 the other respiratory muscles are relaxed, several muscles of 

 the back, abdomen, and chest, by their elastic tonicity, force 

 air again out of the mouth, w T hich completes the process of 

 expiration without expending any nervous energy* and at the 

 same time gives rest to the muscles employed in breathing. 

 Hence the muscles of respiration are not all the time in ac- 



444. W"hat is the substance of the lung composed of? Define pneumonia. 445. What 

 is the principal muscle of respiration? Describe the mechanism of breathing. W'hich 

 act is active and >vhich is passive ? When do the respiratory muscles re-sl? 



