18 DISEASES OF THE HORSE. 
The manner in which the horse coughs is of importance in diagno- 
sis. The cough is a forced expiration, following immediately upon a 
forcible separation of the vocal cords. The purpose of the cough is 
to remove some irritant substance from the respiratory passages, and 
it occurs when irritant gases, such as smoke, ammonia, sulphur vapor, 
or dust, have been inhaled. It occurs from inhalation of cold air if 
the respiratory passages are sensitive from disease. In laryngitis, 
bronchitis, and pneumonia, cough is very easily excited and occurs 
merely from accumulation of mucus and inflammatory product upon 
the irritated respiratory mucous membrane. If one wishes to deter- 
mine the character of the cough, it can easily be excited by pressing 
upon the larynx with the thumb and finger. The larynx should be 
pressed from side to side and the pressure removed the moment the 
horse commences to cough. A painful cough occurs in pleurisy, also 
in laryngitis, bronchitis, and bronchial pneumonia. Pain is shown 
by the effort the animal exerts to repress the cough. The cough is 
not painful, as a rule, in the chronic diseases of the respiratory tract. 
The force of the cough is considerable when it is not especially pain- 
ful and when the lungs are not seriously involved. When the lungs 
are so diseased that they can not be filled with a large volume of air, 
and in heaves, the cough is weak, as it is also in weak, debilitated 
animals. If mucus or pus is coughed out, or if the cough is accom- 
panied by a gurgling sound, it is said to be moist; it is dry when 
these characteristics are not present—that is, when the air in passing 
out passes over surface not loaded with secretion. 
In the examination of the chest we resort to percussion and aus- 
cultation. When a cask or other structure containing air is tapped 
upon, or percussed, a hollow sound is given forth. If the cask con- 
tains fluid, the sound is of a dull and of quite a different character. 
Similarly, the amount of air contained in the lungs can be estimated, - 
by tapping upon, or percussing, the walls of the chest. Percussion is 
practiced with the fingers alone or with the aid of a special percus- 
sion hammer and an object to strike upon known as a pleximeter. 
If the fingers are used, the middle finger of the left hand should be 
pressed firmly against the side of the horse and should be struck with 
the ends of the fingers of the right hand bent at a right angle so as to 
form ahammer. The percussion hammer sold by instrument makers 
is made of rubber or has a rubber tip, so that when the pleximeter, 
which is placed against the side, is struck the impact will not be 
accompanied by a noise. After experience in this method of exami- 
nation one can determine with a considerable degree of accuracy 
whether the lung contains a normal amount of air or not. If, as in 
pheumonia, air has been displaced by inflammatory product occupy- 
ing the air space, or if fluid collects in the lower part of the chest, the 
percussion sound becomes dull. If, as in emphysema, or in pneu- 
