358 RESPIRATION [OH. XXVI. 



slight pause between the end of expiration and the beginning of the 

 next inspiration. 



If the ear is placed in contact with the wall of the chest, or is 

 separated from it only by a good conductor of sound or stethoscope, 

 a faint respiratory or vesicular murmur is heard during inspiration. 

 This sound varies somewhat in different parts being loudest or 

 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 bast 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. 



During the action of the muscles which directly draw air into 

 the chest, those which guard the opening through which it enters are 

 not passive. In hurried breathing the instinctive dilatation of the 

 nostrils is well seen, although under ordinary conditions 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. 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 averages 

 about 500 c.c., or rather more than 30 cubic inches, according to the 

 recent measurements made by Haldane. This will be expanded at 

 body temperature to 600 c.c. This amount of air is not sufficient 

 to fill the lungs. Haldane gives the capacity of the upper 

 air-passages and bronchial tubes as 200 c.c., and therefore about 

 a third of the tidal air is required to fill this dead 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 

 the next inspiration effects a complete mixture of this air with 

 that left in the air-passages; the air in the axial stream of the 

 current will penetrate as far 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 



