Among the birds, however, quite distinct differences in some pathological processes occur, not only from the mammals but also within the class. As a whole birds do not produce pus as we know it in man, probably because of the absence from their leucocytes of a protein- splitting ferment; their leucocyte-producing organs do not seem to respond as readily to a virus, the place of purulent exudate being taken by a coagulum or necrosis. The former varies from a clear gelatin-like material seen upon serous surfaces to a thick mat or mass of coarse but short fibrinous strands. Necrosis may succeed upon the latter or occur so promptly as to appear like the original form of degeneration. It is usually rapid, accompanied by a circumferential congestion but not associated with active phagocytosis. Giant cell production is variable, but when developed the appearance is like that of large syncytia. Hemolysis is not marked in the simple infections but a hyperplasia of the mononuclear nodes of the liver is the rule. The function of this nodal increase is not quite clear. It has been always thought that the scanty bone marrow would supply the necessary erythrocytes, but we have seen these mononuclear areas full of pale red cells fitted with round nuclei and without pigment. The fibrin mentioned above does not have the delicate interweaving that we know in a fibrinous exudate in man. This is interesting when we consider the composition of the blood and its coagulation in the Aves. The cell upon which human coagulation seems to depend, the platelet, is represented in birds by the thrombocyte, which appears only up to about 50,000 per cubic millimetre. Coagulation time is relatively short and the resulting clot is firm and irregular. Perhaps this may have something to do with the nature of an inflammatory exudate. The response to infection on the part of birds may to some extent depend upon differences in anatomy, which are quite distinct, not only from the mammals within which class the anatomy is more uniform, but also from one avian order to another. These differences among the birds may be exemplified by the large foramina between lungs and air sacs in the water birds, a passage which permits infection, notably mycosis, to spread from the first to the second. Again the close apposition of the pancreas to the duodenum over a long stretch permits easy infection of the former from the latter. Still again the large renal-portal vein in the gallinaceous birds explains some of the infections of the liver secondary to intestinal disease. The position of the lungs, deep in the thorax and fitted into recesses made by the sharp anterior border of the ribs and overlaid anteriorly by a rather firm air sac wall, makes it difficult for these organs to expand and therefore renders even a simple congestion a dangerous thing. The position of the ovary subjects the shell-less egg to much danger from the intestinal area.