HEREDITARY DISEASES OF HORSES. 
587 
this than those affecting the respiratory organs, such as 
chronic cough, thick-wind, and roaring, all of which are 
usually hereditary. 
Chronic Cough depends on excessive irritability of the mu- 
cous membrane of the trachea or bronchii, and often occurs as 
a consequence of bronchitis. In unfavorable states of the at- 
mosphere it is greatly aggravated, and in all such cases the 
slightest over-exertion is sufficient to cause a painful, harass- 
ing cough. This irritable condition is very apt to be here- 
ditary. A thickened state of the same mucous membrane, 
inducing thick wind, grunting, and some of the cases con- 
sidered as broken wind, is also hereditary ; and the same 
obtains with roaring , especially that form of it which alone, 
in strict propriety, is entitled to the name, and which consists 
in atrophy, or wasting of some of the muscles of the larynx, 
especially the crico-arytaenoideus posticus. In consequence 
of this wasting, the cartilages of the larynx fall inwards at 
every inspiration, and the consequent diminution of the 
passage through which the air passes causes in respiration a 
peculiar roaring, grating sound. If the ear be applied to the 
throat of a roarer, there may usually be heard at each inspi- 
ration a grating flap, caused by the cartilage falling inwards. 
It is important to observe that this sound, as also the 
roaring noise accompanying it, occurs during Aspiration : 
for when such a sound occurs during aspiration, it indicates 
a morbid condition of the lower parts of the bronchial tubes. 
In roaring, the particular timbre or quality of the sound 
varies considerably, according to the amount of the obstruc- 
tion. Where this is very great, and the diameter of the tube 
much reduced, a sharp whistling noise is produced ; while, if 
there be less diminution in the calibre of the tube, a deep 
roaring or grunting noise is observed. Hence whistling is 
not in all cases, as is generally believed, a less serious, but, 
on the contrary, is frequently a much more serious, affection 
than roaring, resulting, as it often does, from an aggravated 
state of the same morbid condition on which roaring depends. 
Roaring may be readily detected, even when not very bad, 
by giving the animal a gallop, which renders the defect 
apparent by increasing the rapidity"and depth of the respi- 
rations, or by suddenly threatening to strike the animal, or 
giving him a smart blow on the ribs, which causes a sigh or 
deep-drawn inspiration, and thus gives rise to the sound 
characteristic of the disease. Pressure on the larynx also 
induces loud and repeated coughing, and in such cases it is 
observable that each particular cough has a different sound. 
Many cases, illustrating the hereditary nature of roaring, 
