TUBERCULO-INFECTION OF MAN. 
553 
are chiefly particles of fatty matter or minute oil globules, some 
of which are of appreciable size; the greater number, however, 
are immeasurably small. Like the fatty globules suspended in 
milk, they give the chyle a similar “ milky ” aspect; otherwise it is 
identical with lymph. Therefore we can, with Stewart, describe 
the circulation of the food-substances in the economy of the 
higher animals in one phase by saying, the blood feeds the hnyph 
and the lymph feeds the cell, since no blood-vessel is believed to 
enter a cell. The blood contains at one time and another every¬ 
thing that is about to become a part of the tissues, as well as 
everything that has ceased to belong to them. 
Whether the leucocytes play any part in the normal nutrition 
of the other cells is not certainly known, but they have another 
important function which it is necessary to refer to here. 
Phagocytosis .—The phagocytes, i.e., cell-eaters, are certain 
ameboid cells of the blood and lymph which are able to include or 
“ eat up,” devour or absorb foreign bodies with which they come 
in contact, in the same way as the ameba takes in its food. The 
behavior of these phagocytes towards pathogenic micro-organ¬ 
isms, is, to us here, now, of greatest interest and importance. We 
owe our knowledge on this subject to Metchnikoff, who showed 
by his researches on daphnia, a small crustacean with transparent 
tissues observable under the microscope, that when daphnia is fed 
with the spores of a fungus, the monospora, these spores find 
their way into the body cavity of daphnia, where they are at once 
met, attacked, ingested and destroyed by the leucocytes. But—• 
and note this—after a while so many spores get through that the 
leucocytes cannot handle them, at least are not able to deal with 
them all; some of them develop into the first or conidium stage of 
the fungus, and the conidia, instead of being destroyed by them, 
kill the leucocytes, and generally the animal dies. Occasionally, 
however, the leucocytes are able to destroy all the spores, and 
the life of the daphnia is preserved. This battle, ending some¬ 
times in victory, sometimes in defeat, is believed by Metchnikoff 
to be tynical of the struggle which the phagocytes of the higher 
animals, including man. engage in when germs of disease are in¬ 
troduced into the organism. 
