354 RESPIRATION. [CH. xxiv. 



chest, those which guard the opening through which it enters 

 are not passive. In hurried breathing the instinctive dilata- 

 tion of the nostrils is well seen, although under ordinary con- 

 ditions it may not be noticeable. The opening at the upper 

 part of the larynx or rima glottidis is slightly dilated at each 

 inspiration for the more ready passage of air, and. becomes 

 smaller at each expiration; its condition, therefore, corresponds 

 during respiration with that of the walls of the chest. There is 

 a further likeness between the two acts in that, under ordinary 

 circumstances, the dilatation of the rima glottidis is a muscular 

 act and its narrowing chiefly an elastic recoil. 



Terms used to express Quantity of Air breathed. 

 a. Breathing or tidal air is the quantity of air which is habitually 

 and almost uniformly changed in each act of breathing. In a 

 healthy adult man it is about 20 cubic inches, or about 300 ccm. 

 It will be seen that this amount of air is not nearly sufficient to 

 fill the lungs ; it fills the upper respiratory passages ; the air 

 principally finds its way into the alveoli by the much slower 

 process of diffusion, the oxygen diffusing downwards, and the 

 carbonic acid diffusing upwards. Some of the tidal air, however, 

 probably reaches the alveoli directly, for the "respiratory dead 

 space " (the capacity of the upper air passages and the bronchial 

 tubes) is only 140 cc. (Zuntz.) 



b. Complemental air is the quantity over and above this which 

 can be drawn into the lungs in the dedpest inspiration ; its 

 amount varies, but it may be reckoned as 100 cubic inches, or 

 about i,6oo ccm. 



c. Reserve or supplemental air. After ordinary expiration, such 

 as that which expels the breathing or tidal air, a certain quantity 

 of air, about 100 cubic inches (1,600 ccm.) remains in the lungs, 

 which may be expelled by a forcible and deeper expiration. This 

 is termed reserve or supplemental air. 



d. Residual air is the quantity which still remains in the lungs 

 after the most violent expiratory effort. Its amount depends in 

 great measure on the absolute size of the chest, but may be 

 estimated at about 100 cubic inches, or about 1,600 ccm. 



The total quantity of air which passes into and out of the 

 lungs of an adult, at rest, in 24 hours, varies from 400,000 

 (Marcet) to 680,000 (Hutchinsou) cubic inches. This quantity, 

 however, is increased and may be more than doubled by exertion. 



e. Respiratory or Vital Capacity. The vital capacity of the 

 chest is indicated by the quantity of air which a person can expel 

 from his lungs by a forcible expiration after the deepest inspiration 

 possible. The average capacity of an adult, at 15*4 C. (60 F.), 



