=Increase= of resonance without any perceptible modification in
character is usually partial and depends on the increased distension of
the air cells of one lung, or part of a lung, to make up for the loss of
a part or a whole lung through hepatization or pressure by false
membrane or from water in the chest. If a part of a lung is solid and
impervious it gives a dull, dead sound, contrasting strongly with the
increased clearness of the remainder. So with water in the chest, the
clearness of the upper parts contrasts unmistakably with the dullness of
the lower. By watching the advance or retirement of these symptoms the
solidification of a lung and its process of clearing up, and the
effusion of water in the chest and its removal may be equally traced
through all these stages.
If the increased clearness is confined to the upper, lower, or posterior
border of one or both lungs, the sound being natural over all other
parts, it indicates the existence of emphysema of the lungs, a condition
almost constant in broken winded horses.
If the sound is drum-like over most of the lung it is due either to
extensive emphysema or to the presence of air as well as liquid in the
cavity of the chest. In the case first noticed there will be the double
action of the flank, the weak, dry, husky cough and the wheezing
breathing; in the last there will have been the previous attack of
pleurisy, and the application of the ear to the chest will detect a
splashing sound constant or heard only at intervals or on rising. This
should be carefully distinguished from abdominal gurgling.
=Diminished= resonance, noticed over an entire lung, may be due to
congestion or œdema of the lung, to the formation of a thick false
membrane over the inner surface of the ribs or to a false membrane
enveloping the lung and preventing its due distension. Congestion will
be distinguished by the blueness of the mucous membranes and the
presence of a crepitant sound heard on auscultation. Pleurisy is known
by the tenderness on percussion or on pinching the intercostal spaces,
and by the presence in many cases of a friction sound. The sound may be
further lessened in cattle by the deposit of tubercle on the inner side
of the ribs, or the extensive deposition of miliary tubercle throughout
the substance of the lung.
=Absence= of resonance, the sound brought out by percussion being
similar to that obtained by practising it over the muscular masses of
the haunch, is always partial. It is due either to hepatization or to
water in the chest. Hepatization is distinguished by its rarely
affecting the lower thirds of both lungs at once, by the presence of a
crepitating râle round the margin of the area of dullness, and by the
increased resonance and respiratory murmur over the sound parts of the
same and the opposite lung. In water in the chest on the other hand a
friction sound and much tenderness precedes the dullness; the tenderness
continues and the dullness reaches the same height on both sides of the
chest, in the case of the horse. In the ox, water may exist on one side
of the chest only, but the tenderness on pressure and the absence of any
crepitation serve to distinguish the case from pneumonia. In the smaller
animals the position of the dulness may be altered by turning the
patient on its back as the water always gravitates to the lowest point.
The presence of extensive deposits of tubercle, of cretaceous material
in tubercular cows and sheep, and the presence of large cysts in the
lung may give rise to dullness over a circumscribed area. Such areas of
dullness are usually multiple with sound lung between.
A further modification known as the =cracked pot sound= is sometimes
heard in horses and cattle. It may be aptly represented by laying the
palms of the two hands together in such a way that they meet all round
and leave an interval filled with air right in the centre. The back of
the one hand is then struck against the knee when the noise of the air
escaping gives the characteristic sound. It occurs in consumption or in
the advanced stages of inflamed lungs when a large tubercle or abscess
has burst into a bronchial tube and the resulting cavity opens into this
tube by a narrow orifice.