in rare cases.
It is necessary, therefore, in examining blood diseases, to confine
ourselves to the study of the blood-unit, which is always taken as the
cubic millimetre, without reference to the number of units in the body.

Anaemia.

_Anaemia_ is often used as a generic term for all blood diseases, for in
almost all of them the haemoglobin is diminished, either as a result of
diminution in the number of the red corpuscles in which it is contained,
or because the individual red corpuscles contain a smaller amount of
haemoglobin than the normal. As haemoglobin is the medium of respiratory
interchange, its diminution causes obvious symptoms, which are much more
easily appreciated by the patient than those caused by alterations in
the plasma or the leucocytes. It is customary to divide anaemias into
"primary" and "secondary": the primary are those for which no adequate
cause has as yet been discovered; the secondary, those whose cause is
known. Among the former are usually included chlorosis, pernicious
anaemia, and sometimes the leucocythaemias; among the latter, the
anaemias due to such agencies as malignant disease, malaria, chronic
metallic poisoning, chronic haemorrhage, tubercle, Bright's disease,
infective processes, intestinal parasites, &c. As our knowledge
advances, however, this distinction will probably be given up, for the
causes of several of the primary anaemias have been discovered. For
example, the anaemia due to _bothriocephalus_, an intestinal parasite,
is clinically indistinguishable from the other forms of pernicious
anaemia with which it used to be included, and leucocythaemia has been
declared by Lowit, though probably erroneously, to be due to a blood
parasite closely related to that of malaria. In all these conditions
there is a considerable similarity in the symptoms produced and in the
pathological anatomy. The general symptoms are pallor of the skin and
mucous membranes, weakness and lassitude, shortness of breath,
palpitation, a tendency to fainting, and usually also gastro-intestinal
disturbance, headache and neuralgia. The heart is often dilated, and on
auscultation the systolic murmurs associated with that condition are
heard. In fatal cases the internal organs are found to be pale, and very
often their cells contain an excessive amount of fat. In many anaemias
there is a special tendency to haemorrhage. Most of the above symptoms
and organic changes are directly due to diminished res