1170 PHYSIOLOGY 



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 

 overcoming the elastic resistance of the lungs and costal cartilages. 

 A free access of air is provided for by contractions of certain accessory 

 muscles of respiration. 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 inspira- 

 tion and expiration (Fig. 253, p. 583). When inspiration is laboured, 

 the alse 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 r 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 into the spiro- 

 meter. The air left in the lungs after the most vigorous expiration 

 is known as the residual air. 



The residual air may be determined by letting a person expire to the utmost 

 extent and then connecting with his mouth or nose a bag of known capacity 

 filled with hydrogen. The subject of the experiment then inspires and expires 

 into the bag two or three times, ending in the same state of forced expiration 

 as he began. Any diminution of the total volume of gas in the bag will represent 



