MECHANICS OF THE RESPIKATORY MOVEMENTS 1095 



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

 of a new-born mammal ; but this is owing 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. 312, p. 622). 

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

 dilator 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 \ 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 mjay be a positive pressure 

 of -f- 87 mm. 



PULMONARY VENTILATION. Under no circumstances can we by forced 

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

 piration, if the pleura were perforated, the lungs would collapse and driv- 

 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 (complemented 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 

 into the spirometer. 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 the gas lost during the experiment by diffusion 

 into the blood vessels. By analysis of the gaseous mixture in the bag, it is possible to 

 determine the amount of air in the lungs at the beginning of the experiment. Supposing, 

 for example, the bag held 4000 c.c. hydrogen, after two respirations the total volume is 



