DISEASES OF BLOOD AND BONE MARROW 87 
endure a wound and the rapid local pus formation about 
an infection speak for an easy mobilization of their 
cellular defenders ; their connective tissue elements seem 
equally well brought into play. In so far as birds are con- 
cerned perhaps the normally large number of leucocytes 
and the participation of local tissue cells in response to 
irritation is a preparatory protective mechanism because 
of their relatively small amount of bone marrow which 
may not be able to mobilize new cells rapidly ; many of the 
birds, notably those prepared for long flight, have much of 
their osseous system given over to air space. The number 
of red blood cells is also greater in Mammalia, which 
show a variation from 4,000,000 per cubic millimetre in 
some small genera to 12,000,000 per cubic millimetre in 
some ungulates, while birds vary from 2-5,000,000 per 
cubic millimetre. Despite these fundamental diiferences 
in the classes, pathological changes of anemia, leucocy- 
tosis and leucemia are comparable; polycythemia in 
lower animals is unknown to me but may of course occur. 
Anemia. 
As in human pathology this condition may be 
divided into the group that follows some disease which 
damages the red blood cells or their source, called sec- 
ondary, and those cases not preceded by such a condition, 
called primary. Formerly this latter group, known as 
progressive pernicious anemia, was copiously repre- 
sented, but study has discovered that worms, inorganic 
poisons and infections can produce a picture of grave 
anemia so that the formerly large group has dwindled. 
We now conceive a primary anemia to be one without 
discoverable responsible antecedent pathology, therefore 
a disease of the bone marrow itself. There is one variety, 
hemolytic anemia, which seems to be an intoxication of 
the bone marrow with solution of red cells, but the 
affected tissue puts up some struggle against the poison. 
In another primary, the so-called aplastic anemia, no 
