NINTH ANNUAL YEAR BOOK— PART X 41-> 
with the phenomena and locality of the affection, but there is a great 
diversity of opinion as regards the exact cause. Asthma is generally 
thought to be due to spasm of the small circular muscles that surround 
the bronchial tubes. The continued existence of this affection of the 
muscles leads to a paralysis of them and the forced breathing to em- 
physema, which always accompanies heaves. 
Heaves is usually associated v*^ith disorder of the function of diges- 
tion or to an error in the choice of food. Feeding on clover hay or 
damaged hay or straw, too bulky and innutritions food, and keeping the 
horse in a dusty atmosphere or a badly ventilated stable produce or pre- 
dispose to heaves. Horse brought from a high to a low level ate pre- 
disposed. 
In itself broken wind is not a fatal disease but death is generally 
caused by an affection closely connected with it. After death, if the 
organs are examined, the lesions found depend much upon the length of 
time broken wind has affected the animal. In recent cases very few 
changes are noticeable, but in animals that have been broken-winded for 
a long time the changes are well marked. The lungs are paler than 
natural, and of much less weight in proportion to the volume, as evi- 
denced by floating them in water. The walls of the small bronchial 
tubes and the membrane of the larger tubes are tickened. The right 
side of the heart is enlarged and its cavities dilated. The stomach is 
enlarged and its walls stretched. The important change found in the 
lungs is a condition technically called pulmonary emphysema. This is 
of two varieties: First, w^hat is termed vesicular emphysema, which con- 
sists of an enlargement of the capacity of the air cells (air vesicles) by 
dilation of their walls. The second form is called interlobular, or inter- 
stitial, emphysema, and follows the first. In this variety the air finds 
its way into the lung tissue between the air cells or the tissue between 
the small lobules. 
8ymi)toms.— Almost every experienced horseman is able to detect 
heaves. The peculiar movement of the flanks and abdomen point out the 
ailment at once. But in recent cases the affected animal does not always 
exhibit the characteristic breathing unless exerted to a certain extent. 
The cough which accompanies this disease is peculiar to it. It is diffi- 
cult to describe, but the sound is short, and something like a grunt. When 
air is inspired— that is, taken in— it appears to be done in the same 
manner as in health; it may possibly be done a little quicker than 
natural, but not enough to attract any notice. It is when the act of ex- 
piration (or expelling the air from the lungs) is performed that the 
great change in the breathing is perceptible. It must be remembered 
that the lungs have lost much of their elasticity, and in consequence, of 
their power of contracting on account of the degeneration of the walls of 
the air cells, and also on account of the paralysis of muscular tissue be- 
fore mentioned. The air passes into them freely, but the power to expel 
it is lost to a great extent by the lungs; therefore the abdominal muscles 
are brought into play. These muscles, especially in the region of the 
flank, are seen to contract, then pause for a moment, then complete the 
act of contracting, thus baking a double bellowslike movement at each 
