34 Organs of the Animal Body : their Forms and Uses. 
being counted as one, fifteen times in a minute. A horse in 
the same state breathes eight to ten times in a minute. An 
ox is said to breathe fifteen to twenty-five times in a minute ; 
but if the farmer will do as the writer has often done, county the 
breathings of cattle when at rest at different times of the year, 
he will find that in hot weather, or in winter in a warm shed, 
thirty, forty, or fifty breathings in a minute are not uncommon ; 
and he will very likely be as much surprised as the writer was to 
find healthy sheep breathing from thirty to two hundred times in 
a minute when at rest. Pigs seem to maintain a tolerably 
steady rate of breathing from fifteen to twenty-five times in the 
minute. Great differences in the number of breathings are 
therefore to be expected in farm-stock in good health. 
Of more consequence than the number of breathings in a 
minute is the mode in which the act is done, and above all 
is the result of the process to the animal body. If the farmer 
will look at the carcass from which he has taken the lungs, he 
will note that he has cut through a good many muscles in 
taking off the wall of the chest, and at the back of the cavity 
which contained the lungs he will see projecting forwards a 
partition of red muscle and white glistening fibre which sepa- 
rates the chest from the cavity behind it (abdomen). He may 
recognise the muscular partition as the skirt ; the anatomist 
calls it the diaphragm. 
In the act of breathing-in the air (inspiration) the walls of 
the chest are pulled upwards and outwards by the contraction 
of muscles attached to the ribs, and at the same time the 
diaphragm contracts and pulls the centre, which projects into 
the chest, backwards, and thus the space in the chest is much 
increased. The pressure of the air in the chest is now less than 
that outside, and as a natural result the outside air rushes into 
the nostrils or mouth down the windpipe into the lungs. 
When the lungs are filled, the muscles cease to contract. 
The walls of the chest and the diaphragm go back to their 
former position, and the excess of air is pressed out, the 
expulsion being aided by the contraction of the muscles of the 
flanks. But it is important to inquire what has happened 
during the entrance and exit of air. 
First, it must be remembered that the lungs are not emptied 
of air by the act of breathing out ; in fact they are always fairly 
full of air, and the air in the lungs is always giving up some 
of its oxygen to the blood, and getting back some carbonic acid 
from the blood, and some watery vapour and certain worn-out 
materials in exchange — a bad exchange for the air in the 
lungs, but a very good one for the blood, which becomes again 
fit for use in consequence. 
