CH. XXIY.] TIDAL AIR 357 



coarsest in the neighbourhood of the trachea and large bronchi 

 (tracheal and bronchial breathing), and fading off into a faint sighing 

 as the ear is placed at a distance from these (vesicular breathing). It 

 is best heard in children, and in them a faint murmur is heard in ex- 

 piration also. The cause of the vesicular murmur has received various 

 explanations ; but most observers hold that the sound is produced by 

 the air passing through the glottis and larger tubes, and that this 

 sound is modified in its conduction through the substance of the lung. 

 The alterations in the normal breath sounds, and the various additions 

 to them that occur in different diseased conditions, can only be 

 properly studied at the bedside. 



Respiratory movements of the Nostrils and of the Glottis. During 

 the action of the muscles which directly draw air into the chest, 

 those which guard the opening through which it enters are not pas- 

 sive. In hurried breathing the instinctive dilatation of the nostrils 

 is well seen, although under ordinary conditions it may not be notice- 

 able. 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 ordi- 

 nary 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. 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 c.c. It will be seen that this amount 

 of air is not nearly sufficient to fill the lungs; it fills the upper 

 respiratory passages ; Zuntz gives the capacity of the upper air 

 passages and bronchial tubes as 140 c.c., and if this low estimate is 

 correct, about half the tidal air is required to fill this space. At the 

 end of an expiration, however, the tubes and alveoli are not empty 

 of air, and the sudden inrush of atmospheric air during inspiration 

 effects a complete mixture of this air with that left in the air 

 passages ; it is possible that the air in the axial stream of the current 

 may penetrate as far even as the alveoli, but what is sucked into the 

 alveoli is mainly some of the mixture from the bronchial passages, 

 and that in turn is derived from the mixture (containing more atmos- 

 pheric air in proportion) in the upper air cavities. During expiration 

 the air which leaves the lungs may come in part from the alveoli, but 

 the effect of the stream of outgoing air is mainly as before to effect a 

 thorough admixture of the air in the intermediate air passages ; thus 

 the alveolar air will become mixed with that in the bronchial tubes, 

 and that in turn will be mixed with that in the upper air chambers. 

 In a succession of alternate inspirations and expirations adequate 



