128 CLINICAL DIAGxVOSTICS. 



The normal pulmonary percussion sound is due to the 



iound of the blow, the vibration of the thoracic wall and of 

 the air contained in the lung. In large animals the sound is 

 generally quite dull (subdued), but clearer in the middle and 

 lower portions of the chest. In small animals the sound is 

 about the same over the whole field. The thinner the chest 

 wall, the clearer the sound. Toward the borders of the thoracic 

 cavity the intensity of the lung sound is diminished. 



The pulmonary sound is heard over an area larger than 

 normal in vesicular emphysema, in the rarer interstitial em- 

 physema and in pneumothorax, because in these conditions 

 the diaphragm is forced backward. In vicarious emphysema 

 s similar increase in area is sometimes noted. 



The pulmonary percussion sound (pulmonary resonance) 

 is abnormally loud and clear ("over-loud"), due to disease: 



1. If the lung is greatly inflated (emphysema). 



2. If the lung is abnormally distended with air as it 

 occurs, for instance, at the border of pleural exudate. 



3. In pneumothorax. 



The tympanitic percussion sound is noted over the 

 chest when the vibrating column of air, whether small or 

 large, is surrounded by a rigid wall and there is communica- 

 tion with the outside world. In small animals, therefore, the 

 pulmonary percussion sound is normally tympanitic. There is 

 no marked boundary of distinction between the tympanitic and 

 the full pulmonary percussion sound. A tympanitic tone is 

 heard : 



1. When a portion of the lung containing air is mors 

 or less surrounded by solidified tissue or exudate, which 

 isolates it from its environment. 



Therefore : 



In the beginning and last stages of fibrinous pneumonia. 



In catarrhal pneumonia. 



In pulmonary edema and in pulmonary atelectasis. 



