RESPIRATION. 183 



intercostals, the triangularis sterni, the serratus posticus inferior, and 

 quadratus lumborum. When by the efforts of the expiratory muscles, 

 the chest has been squeezed to less than its average diameter, it again, on 

 relaxation of the muscles, returns to the normal dimensions by virtue of 

 its elasticity. The construction 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 

 cohesion as well as 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 

 or stethoscope, a faint respiratory murmur is heard during inspiration. 

 This sound varies somewhat in different parts being loudest or coarsest 

 in the neighborhood 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 expiration also. 

 The cause of the vesicular 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 

 distention (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 produced in the glottis, but that it is modified in its 

 passage to the pulmonary alveoli (Beau, Gee). 



Respiratory Movements of the Nostrils and of the Glottis. 



