614 MR. YOUATT S VETERINARY LECTURES. 
fibres, is to lessen the convexity towards the chest, and the con¬ 
cavity towards the abdomen : perhaps, by a powerful contraction, 
to cause it to present a plane surface either way. The abdominal 
viscera that must be displaced in order to effect this, have con¬ 
siderable bulk and weight; and when the stomach is distended 
with food, and the motion Required from the diaphragm in rapid 
breathing is both quick and extensive, there needs some strong, 
firm, elastic, substance to bear it. The forcible contact and violent 
pressure would bruise and otherwise injure a mere muscular expan¬ 
sion ; and therefore we have this tendinous expansion, compara¬ 
tively devoid of sensibility, to stand the pressure and the shock, 
which will always be greatest at the centre. 
The Centre of Expansion in other Animals.—The Ox. —Now 
compare this expansion in the horse and the ox, the one formed 
for speed and the other for slow work only, and for the accumu¬ 
lation of flesh and fat. See the wide expanse of tendon in the 
horse ; the comparatively little development in the ox. See also 
the thickness of muscle in the ox, compared with that in the horse. 
It may be placed here in him with safety; there will seldom be 
rapid action, and his work will be more easily done and with less 
exhaustion; yet occasionally, from the weight of his paunch, 
pressing into the concave portion of the diaphragm, the labour 
will be considerable, although slow. 
The T)og. —Look again at the dog. The muscular part of the 
diaphragm is thick and strong, the aponeurotic expansion com¬ 
paratively smaller. From the smaller expanse of the thorax in 
this animal, and the consequent little expansion of the diaphragm, 
the action, although occasionally rapid and violent (for he is an 
animal of speed), is not so extensive; and more muscle and less 
tendon may be given to him, not only without detriment, but 
with evident advantage. Therefore it is, that although we have 
occasional rupture of the heart in the dog, oftener perhaps than 
in the horse, we have no rupture of the diaphragm, at least there 
is no case of it on record, and no one has come under my 
observation. In the ox I never saw rupture of the heart, and 
rarely that of the diaphragm ; and for very sufficient reasons, 
whether we regard the structure or the habits of the animal. 
Spasm of the Diaphragm. — When I look at this large apo¬ 
neurotic expansion and comparative little muscular power in the 
diaphragm of the horse, and think of the frequent thoughtless 
and cruel exaction of labour from this portion of the respiratory 
machine, I cease to wonder at that which was first hinted at by 
a sporting writer of deserved celebrity, and afterwards so well 
described by that accurate observer Mr. Castley. A horse had 
been “ a good deal blown” in a run of an hour without a check. 
