SECTION IV 
DISEASES OF THE BLOOD AND BONE MARROW 
The production and physiology of the circulating blood 
seem closely similar in the two classes under considera- 
tion, although the anatomy is not the same in birds and 
mammals, variations also occurring within the latter 
group. Pathological responses follow comparable lines in 
that hemolyzing agencies, be they hemosporidia, absorp- 
tions from metazoan parasites or bacterial toxins, produce 
a reaction in erythropoietic centres, and positively chemo- 
tactic viruses call forth increases in the colorless 
elements. We have also observed a decrease of leuco- 
cytes in an Orang Utan suffering from influenza, a finding 
analogous to that in the human attack. There is, how- 
ever, a much less ready response on the part of birds to 
any leucocyte-stimulating influence, in this class the 
mononuclears seeming to bear much of the burden 
assumed by the myeloid cells of Mammalia or at least 
appearing on the stage very quickly so that any increase 
of the latter is overshadowed by them. Perhaps this 
apparent difference may be further explained by the 
greater number of colorless blood cells, structures which 
might be called the principal secondary defences of the 
body and constantly at the disposal of the organism, 
normally present in the birds' blood; they amount to 
25,000 per cubic millimetre in birds, while in the mammals 
very few varieties have half this number. On the accom- 
panying Table (6) will be found a few differential leuco- 
cyte counts now knowm to us. 
It would seem, from a general observation of simple 
and infected wounds and from a few blood counts, that 
the response of leucocytes in the lower animals is greater 
than in monkeys and man. The ease with which animals 
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