AUSCULTATION. 523 



of natural sounds, another series have to be mentioned, which 

 are essentially distinct from the sounds of health ; these are 

 called rales or rattles. They are called dry or humid, 

 according as they convey the idea of air passing through a 

 dry tube or one containing liquid. The dry rales depend 

 on a narrowing of the bronchial tubes, trachea, or larynx, 

 from the deposition of a viscid mucus in layers over the 

 mucous membrane. The shrillness of the sound depends on 

 the narrowing of the tube, and the dry rales have accord- 

 ingly been referred to two principal standards, the sonorous 

 and the sibilant. The sounds of the subcrepitant rales con- 

 vey the idea of bubbles of air bursting in passing through a 

 liquid. They vary necessarily with the size of the bubbles 

 from which they are produced. The chief varieties are the 

 mucous, the crepitant, and the subcrepitant. 



The sonorous rale has been likened to the humming of a 

 gnat, the cooing of a wood-pigeon, or the bass notes of a 

 violin. From these supposed resemblances it will be inferred 

 that it is liable to considerable modification of tone. The 

 deepest or gravest tones are necessarily formed in the largest 

 tubes. It frequently results in the early stages of bronchitis 

 from swelling of the mucous membrane. It is usually very 

 transient, from the mucus which causes it being removed 

 by coughing; but even when permanent, does not last over 

 two or three days, when it is replaced by the mucous rale. 

 In the majority of cases, too, it is very irregular, and can 

 seldom be heard with every successive respiratory movement. 

 It is sometimes accompanied by the sibilant or mucous rales, 

 the former dependent on the smaller tubes being in a similar 

 condition, and the latter on some portion of the tubes having 

 commenced to secrete actively. The sonorous is sometimes 

 modified so as to form what is known as the dry mucous 

 rdle. In this case a piece of tenacious mucus is attached to 



