piratory
interchange from the loss of haemoglobin, and to its effect on the
various organs involved. The diagnosis depends ultimately in all cases
upon the examination of the blood.

Though the relative proportions of the leucocytes are probably
continually undergoing change even in health, especially as the result
of taking food, the number of red corpuscles remains much more constant.
Through the agency of some unknown mechanism, the supply of fresh red
corpuscles from the bone-marrow keeps pace with the destruction of
effete corpuscles, and in health each corpuscle contains a definite and
constant amount of haemoglobin. The disturbance of this arrangement in
anaemia may be due to loss or to increased destruction of corpuscles, to
the supply of a smaller number of new ones, to a diminution of the
amount of haemoglobin in the individual new corpuscles, or to a
combination of these causes. It is most easy to illustrate this by
describing what happens after a haemorrhage. If this is small, the loss
is replaced by the fully-formed corpuscles held in reserve in the
marrow, and there is no disturbance. If it is larger, the amount of
fluid lost is first made up by fluid drawn from the tissues, so that the
number of corpuscles is apparently diminished by dilution of the blood;
the erythroblasts, or formative red corpuscles, of the bone-marrow are
stimulated to proliferation, and new corpuscles are quickly thrown into
the circulation. These are apt, however, to be small and to contain a
subnormal amount of haemoglobin, and it is only after some time that
they are destroyed and their place taken by normal corpuscles. If the
loss has been very great, nucleated red corpuscles may even be carried
into the blood-stream. The blood possesses a great power of recovery, if
time be given it, because the organ (bone-marrow) which forms so many of
its elements never, in health, works at high pressure. Only a part of
the marrow, the so-called red marrow, is normally occupied by
erythroblastic tissue, the rest of the medullary cavity of the bones
being taken up by fat. If any long-continued demand for red corpuscles
is made, the fat is absorbed, and its place gradually taken by red
marrow. This compensatory change is found in all chronic anaemias, no
matter what their cause may be, except in some rare cases in which the
marrow does not react.

It is often very difficult, especially in "secondary" anaemias, to say
which of the above proces