516 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



quiet expiration no muscular factors are involved, unless it be the contraction 

 of the intercostales interni interossei, in which event they are more probably 

 engaged in maintaining the tension of the intercostal tissues than in actually 

 diminishing the capacity of the thorax. 



During forced expiration the abdominales flex the thorax upon the pelvis, 

 force the abdominal viscera against the diaphragm, thrusting it upward, and 

 by pulling upon the lower margins of the thoracic cage draw them inward 

 and at the same time offer a base from which the intercostales interni inter- 

 ossei act to pull the ribs downward; the triangulares sterni contract at the 

 same time and pull downward the cartilages of the second to the sixth ribs, 

 inclusive. 



Associated Respiratory Movements. Associated with the thoracic and 

 abdominal movements of respiration are movements of the face, pharynx, and 

 larynx. The nostrils are slightly dilated during inspiration and passively 

 return to their condition of rest during expiration ; the soft palate moves to 

 and fro with the inflow and outflow of air, and the glottis is widened during 

 inspiration and narrowed during expiration. During labored inspiration, 

 besides the above movements, the mouth is usually opened ; the muscles con- 

 cerned in facial expression may be active, giving the individual an appearance 

 of distress ; the soft palate is raised, and the larynx descends. The widening 

 of the nares and the glottis, the opening of the mouth, the elevation of the soft 

 palate, and the descent of the larynx during inspiration are obviously for the 

 purpose of lessening the resistance to the inflow of air. 



Intrapulmonary or Respiratory Pressure and Intrathoracic Pressure. 

 The tidal flow of air to and from the lungs during the respiratory move- 

 ments is due, as already stated, to the differences between the pressure within 

 the lungs and that outside the body. During inspiration the enlargement of 

 the thorax causes an expansion of the lungs and a consequent diminution of 

 pressure within them, so the air is forced through the air-passages until the 

 pressure within the lungs equals that of the atmosphere; during expiration 

 there occur elastic and mechanical reactions whereby the pressure within the 

 lungs is greater than that of the atmosphere, consequently air is expelled until 

 an equilibrium is again established. It is apparent, then, that during inspira- 

 tion there exists within the lungs a condition of negative pressure, and that 

 during expiration the pressure is positive. If a manometer be so arranged as 

 in no way to interfere with the ingress and egress of air, it will be found that 

 during inspiration the column of mercury sinks, while during expiration it 

 rises. Donders found by connecting a manometer with the nasal passage that 

 the pressure during quiet inspiration was 1 millimeter of Hg, and during 

 expiration + 2 to 3 millimeters. Ewald gives as corresponding values 0.1 

 millimeter and +0.13 millimeter, and Mundhorst, 0.5 millimeter and +5 

 millimeters. During deep inspiration Donders noted a pressure of 30 milli- 

 meters, and when the mouth and nose were closed, 57 millimeters. During 

 forced expiration, with respiratory passage closed, it was +87 millimeters ; but 

 these figures have been exceeded. 



