I 
500 JACOB HELMER. 
Resonance is divided into two kinds, pulmonary and tym¬ 
panitic. Pulmonary resonance is that obtained by percussing 
over a healthy lung. Tympanitic resonance is heard when 
there is air inside of a cavity. Jt is a ringing metallic sound 
when caused in this manner. In percussion, sounds are in¬ 
creased, diminished or absent. In the degree that it is di¬ 
minished it is a dull sound. If absent it constitutes flatness. 
Flatness is due to effusion of fluid into a cavity. Pulmonary 
resonance is increased in left lung if the right one is closed. 
As the sound depends on the amount of air in the tissues 
when one part of the lung is blocked, adjacent parts will be¬ 
come more resonant. Pulmonary resonance is diminished 
when there is less than a normal quantity of blood present, 
or where there is exudate, or where there is an increased 
amount of blood in a part, as in congestion of the lungs, and 
in effusion into the interlobular connective tissue. 
Having now considered the various methods of physical 
examination and described the physical signs afforded by 
them, let us give attention to a brief description of the mor¬ 
bid anatomy of the most common pulmonary diseases with 
the physical signs found connected with it. Then we shall 
see clearly how to make a differential diagnosis of these con¬ 
ditions. 
Pneumonia .—This disease is divided into three classes : 
croupous, catarrhal and interstitial. Croupous pneumonia is 
the one we commonly meet. It is an acute inflammation of 
the vesicular structure of the lung followed by temporary 
blocking of the air cells with exudate and exclusion of air. 
In the first stage there is an increased amount of blood in the 
capillary vessels. This causes pressure on the air cells and 
diminishes their calibre; subsequently comes dryness, then 
secretion of plastic material blocks the air cells. The tissue 
becomes the color of liver, being engorged with bloody sero¬ 
fibrinous exudate. The third stage, or that of resolution, is 
marked by the material becoming milky white ; there is now 
again some air in the cells. Absorption goes forward. The 
physical signs of croupous pneumonia in the first stage ap¬ 
pear in about twenty-four hours. If the disease is anterior * 
