e way that it is sucked into a pair of
bellows when expanded. Then the contraction of another set of muscles
decreases the size of the thoracic cavity, and the air is squeezed out
again. The action is just as truly mechanical as is that of the
blacksmith's bellows.

The relation of the air to the blood is just as simple. In the blood
there are various chemical ingredients, among which is one known as
haemoglobin. It does not concern us at present to ask where this material
comes from, since this question is part of the broader question, the
origin of the machine, to be discussed in the second part of this work.
The haemoglobin is a normal constituent of the blood, and, being red in
colour, gives the red colour to the blood. This haemoglobin has peculiar
relations to oxygen. It can be separated from the blood and experimented
upon by the chemist in his laboratory. It is found that when haemoglobin
is brought in contact with oxygen, under sufficient pressure it will
form a chemical union with it. This chemical union is, however, what the
chemist calls a loose combination, since it is readily broken up. If the
oxygen is above a certain rather low pressure, the union will take
place; while if the pressure be below this point the union is at once
destroyed, and the oxygen leaves the haemoglobin to become free. All of
this is a purely chemical matter, and can be demonstrated at will in a
test tube in the laboratory. But this union and disassociation is just
what occurs as the foundation of respiration. The blood coming to the
lungs contains haemoglobin, and since the oxygen pressure in the air is
quite high, this haemoglobin unites at once with a quantity of oxygen
while the blood is flowing through the air-vessels. The blood is then
carried off in the circulation to the active tissues like the muscles.
These tissues are constantly using oxygen to carry on their life
processes, and consequently at all times use up about all the oxygen
within their reach. The result is that in these tissues the oxygen
pressure is very low, and when the oxygen-laden haemoglobin reaches them
the association of the haemoglobin with oxygen is at once broken up and
the oxygen set free in the tissue. It passes at once to the lymph, from
which the active tissues seize it for the purpose of carrying on the
oxidizing processes of the body. This whole matter of supplying the body
with oxygen is thus fundamentally a chemical one, controlled by chemical
l