154 ANIMAL PHYSIOLOGY. 
reason to believe that the red cells do not live longer than a 
few weeks at most. The red cells, in various degrees of disor- 
ganization, have been seen within the white cells (phagocytes), 
and the related cells of the spleen, liver, bone-marrow, etc. In 
fact, these cells, by virtue of retained ancestral (ameboid) quali- 
ties, have devoured the weakened, dying red cells. It seems to 
be a case of survival of the fittest. It is further known that 
abundance of pigment containing iron is found in both spleen 
and liver; and there seems to be no good reason for doubting 
that the various pigments of the secretions of the body (urine, 
bile, etc.) are derived from the universal pigment of the blood. 
These coloring matters, then, are to be regarded as the excreta 
in the first instance of cells behaving like amceboids, and later 
as the elaborations of certain others in the kidney and else- 
where, the special function of which is to get rid of waste 
products. The birth-rate and the death-rate of the blood- 
cells must be in close relation to each other in health; and 
some of the gravest disturbances arise from decided changes 
in the normal proportions of the cells (anemia, leucocythe- 
mia). 
Bath the red and white corpuscles show, like all other cells 
of the organism, alterations corresponding to changes in the 
surrounding conditions. The blood may be withdrawn and its 
cells more readily observed than those of most tissues; so that 
the study of the influence of temperature, feeding of the leuco- 
cytes, and the action of reagents in both classes of cells is both 
of practical importance and theoretic interest, and will well re- 
pay the student for the outlay in time and labor, if attention is 
directed chiefly to the results and the lessons they convey, and 
not, as too commonly happens, principally to the methods of 
manipulation. 
The Chemical Composition of the Blood.—Blood has a decided 
but faint alkaline reaction, owing chiefly to the presence of 
sodium biphosphate (Na,HPO,), a saline taste, and a faint odor 
characteristic of the animal group to which it belongs, owing 
probably to volatile fatty acids. The specific gravity of blood 
varies between 1045 and 1075, with a mean of 1055; the spe- 
cific gravity of the corpuscles being about 1105 and of the 
plasma 1027. This difference explains the sinking of the cor- 
puscles in blood withdrawn from the vessels and kept quiet. 
Much the same difficulties are encountered in attempts at the 
exact determination of the chemical composition of the blood, 
as in the case of other living tissues. Plasma alters its phys- 
