1046 PHYSIOLOGY 



pressure in the pleura may amount to 30 mm. Since the lungs are always 

 tending to collapse, respiration becomes impossible directly free openings 

 are made into the pleural cavities on both sides. With each inspiratory 

 movement air rushes in through these openings, so that the thoracic move- 

 ments can no longer exert any influence on the volume of the lungs. The 

 negative pressure in the thorax is diminished by any factor decreasing the 

 elasticity of the lung- tissue. Thus in an old man, where the elastic tissue is 

 degenerated and the alveoli are enlarged, giving rise to the condition known 

 as emphysema, the lungs may collapse only slightly or not at all on opening 

 the chest. The lungs do not collapse on making an opening in the chest 

 of a new-born mammal ; but this is owin? to the fact that they completely 

 fill the thorax in the expiratory position, and it is only later that, with the 

 growth of the ribs, the thorax gets, so to speak, too large for the lungs, which 

 are therefore stretched to fill it. 



The force exerted by the inspiratory muscles is nearly all spent in over- 

 coming the elastic resistance of the lungs and costal cartilages. A free access 

 of air is provided for by contractions of certain accessory muscles of respira- 

 tion. With each inspiration the glottis is widened by abduction of the vocal 

 cords. When the glottis is observed by means of the laryngoscope, a 

 rhythmical separation and approximation of the vocal cords are observed, 

 synchronous respectively with inspiration and expiration (Fig. 255, p. 524). 

 When inspiration is laboured, the alae nasi are dilated by the action of the 

 dilatator nasi. This movement of the nostril, which is constant in many 

 animals, becomes very marked in children suffering from any respiratory 

 trouble. 



If a manometer be connected with one of the nostrils, so as to register the 

 pressure in the air-cavities, it is found that there is a negative pressure of 

 1 mm. Hg. with inspiration, and a positive pressure of 2 or 3 mm. with 

 expiration. With forced inspiration the negative pressure may amount 

 to 57 mm. Hg., and with forced expiration there may be a positive pressure 

 of + 87 mm. 



PULMONARY VENTILATION. Under no circumstances can we by 

 forced expiration empty the lungs of air. At the end of the most forcible 

 expiration, if the pleura were perforated, the lungs would collapse and drive 

 more air through the trachea. When breathing quietly a man takes in and 

 gives out at each breath about 500 c.c. of air, measured dry and at C. If 

 measured moist and at the temperature of the body, viz. 37 C., the 

 volume would be about 600 c.c. This amount is known as the tidal air. 

 By means of a forcible inspiratory effort it is possible to take in about 1500 

 c.'c. more (complemental air). At the end of a normal expiration a forcible 

 contraction of the expiratory muscles will drive out about 1500 c.c. more 

 (supplemental air). These three amounts together constitute the ' vital 

 capacity ' of an individual. This total may be determined by means of the 

 instrument known as the spirometer, which is merely a small gas-meter with 

 a gauge by which the amount of air in it can be at once read off. The person 

 to be tested fills his lungs as full as possible, and then expires to the utmost 



