188 HAND-BOOK OF PHYSIOLOGY. 



struction of the chest-walls, therefore, admirably adapts them for recoiling 

 against and resisting as well undue contraction as undue dilatation. 



In the natural condition of the parts, the lungs can never contract to 

 the utmost, but are always more or less "on the stretch/' being kept 

 closely in contact with the inner surface of the walls of the chest by 

 atmospheric pressure, and can contract away from these only when, by 

 some means or other, as by making an opening into the pleural cavity, or 

 by the effusion of fluid there, the pressure on the exterior and interior of 

 the lungs becomes equal. Thus, under ordinary circumstances, the 

 degree of contraction or dilatation of the lungs is dependent on that of 

 the boundary walls of the chest, the outer surface of the one being in 

 close contact with the inner surface of the other, and obliged to follow it 

 in all its movements. 



Respiratory Rhythm. The acts of expansion and contraction of 

 the chest, take up, under ordinary circumstances, a nearly equal time. 

 The act of inspiring air, however, especially in women and children, is a 

 little shorter than that of expelling it, and there is commonly a very 

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

 inspiration. The respiratory rhythm may be thus expressed: 



Inspiration 6 



Expiration 7 or 8 



A very slight pause. 



Respiratory Sounds. If the ear be placed in contact with the wall 

 of the chest, or be separated from it only by a good conductor of sound, 

 a faint respiratory murmur is heard during inspiration. This sound 

 varies somewhat in different parts being loudest or coarsest in the neigh- 

 borhood of the trachea and large bronchi (tracheal and bronchial breath- 

 ing), 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 expiration also. The cause of the vesic- 

 ular murmur has received various explanations. Most observers hold 

 that the sound is produced by the friction of the air against the walls of 

 the alveoli of the lungs when they are undergoing distension (Laennec, 

 Skoda), others that it is due to an oscillation of the current of air as it 

 enters the alveoli (Chauveau), whilst others believe that the sound is pro- 

 duced in the glottis, but that it is modified in its passage to the pulmo- 

 nary alveoli (Beau, Gee). 



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 passive. 

 In hurried breathing the instinctive dilatation of the nostrils is well seen, 

 although under ordinary conditions it may not be noticeable. The open- 

 ing at the upper part of the larynx, however, or rima glottidis (Fig. 297), 



