178 
ANIMAL PATHOLOGY. 
passages, and, consequently, the quantity of air which is admitted 
to various parts of the apparatus. 
Roaring is, in the great majority of cases, a disease of the 
larynx ; a diminution of the aperture of the larynx, from occa- 
sional spasm, or malformation, or mechanical obstruction. The 
horse is not distressed in his ordinary breathing ; but when he is 
called upon for any sudden or unusual exertion of speed, and a 
proportionately greater quantity of air is required in order to 
supply the demand of arterial blood, then the difficulty which is 
opposed to its entrance is sufficiently evident in the rushing 
roaring sound which is heard, and the distress which the coun- 
tenance of the animal expresses. What does the owner or the 
driver then do ? He cannot make the horse sound again, — he 
cannot enable him to perform the labour of a sound horse ; but 
he can relieve him from much annoyance, and enable him to do 
ordinary or somewhat more than ordinary work without great 
inconvenience. He ties a strap or a cloth round the nostrils of 
the horse ; in the language of Mr. Field, “ he adjusts the quan- 
tity of air received through the nostrils to the capability of the 
larynx to transmit it to the lungs and thus, by removing the 
irritation and the spasm which the pressure of the accumulating 
air on the constricted part occasioned, he enables the horse — 
not, I repeat it, to do the work of a sound horse, but to do a 
great deal more work than he otherwise could. If more blood is 
not arterialized, and the full muscular exertion restored, yet a 
source of annoyance and oppression, under which little exertion 
could be made, is removed, and the animal breathes with com- 
parative freedom, and works with comparative ease. This, al- 
though applying to another division of the respiratory canal, 
shews the advantage or absolute necessity of sympathy and con- 
sentaneous action between every part. But we must proceed. 
The Bronchi . — When the trachea has entered the chest, be- 
tween the two first ribs, and has reached the third dorsal vertebra, 
it divides, and again subdivides, perforating every part of the 
lungs, until each ramification has become a perfect capillary 
tube, terminating in a minute sac or vesicle. It is of these 
tubes, and their vesicular terminations, that the cellular tissue of 
the lungs consists. At first the bronchi appear to be constructed 
like the trachea ; they have the same cartilaginous rings, but 
the transverse muscle is no longer to be perceived. The carti- 
laginous portions gradually undergo considerable change. They 
lose their circular form — two or three pieces unite to constitute 
each ring — they become irregular in their shape, and at length 
they disappear altogether. The terminating sacs seem to be com- 
posed of gossamer membrane, and between and around each of 
