620 MR. youatt’s veterinary lectures. 
contraction. In their usual state the air-tubes are distended 
beyond their natural calibre. The proof of this is, that if the 
parietes of the thorax are perforated, and the pressure of the 
atmosphere is thus rendered equal within and without them, the 
lungs immediately collapse. Their elasticity is then no mean 
agent in the act of expiration, and even in the contraction of the 
abdominal muscles this principle has much to do. 
Its Effect on the Heart. —This constant effort at contraction 
in the lungs answers another very important purpose. The heart 
is inclosed in the thoracic cavity. It is exposed to the pressure 
of the atmospheric air, which, bearing with its full force on this 
organ, would not permit it to beat; but just in proportion to the 
resiliency or elastic resistance to expansion in the cartilaginous 
or cellular texture of the lungs, the weight or pressure upon the 
heart is taken away or neutralized. The resiliency of the lungs 
and the pressure of the external air are probably more nearly ba¬ 
lanced than we sometimes imagine. 
Here we will at present leave the mere mechanism of respiration. 
This muscle having so much to do, must be occasionally subject 
to injury. I have hinted at that spasmodic action which is often 
the morbid consequence of exhaustion ; and I would again urge 
you to be on your guard that you do not confound it with cardi¬ 
tis, a disease proceeding from a very different cause, and requir¬ 
ing almost opposite treatment. 
Inflammation of the Diaphragm. —The French writers speak 
of inflammation of the diaphragm. I can conceive such a disease 
to be not only a possible but probable occurrence; but I have 
not met with it:—there is no English record of it, and I must 
be content, unwillingly so, with their account of the matter. 
They speak of fever, cough, difficulty of respiration, convulsive 
beating of the flanks, cerebral irritation, convulsions, fury, and 
the last to such an extent as to be confounded with rabies. 
Every case was unsuccessful, in spite of the most vigorous treat¬ 
ment. 
Rupture of the Diaphragm.— Of an accident to which this 
muscle is exposed, namely rupture, we unfortunately know more. 
There are cases on record of ruptured diaphragm in the human 
being, but principally as connected with laborious parturition, 
the rationale of which it is easy to understand. In the horse it 
is the usual consequence of over-exertion. Mr. Cartwright, I 
think, mentions a case in which it occurred in the mare at the 
time of parturition. 
A Case of fatal Rupture. —Mr. Hales, of Oswestry, records 
a case in which a mare driven long at a quick pace suddenly 
shewed symptoms of distress; it was with great difficulty that 
