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276 THE BLOOD.
The constituent of the blood-corpuscles existing in greatest quantity
is the red pigment haemoglobin.
Blood-pigments.
According to Hoppe-Seyler the coloring-matter of the red blood-
corpuscles is not in a free state, but combined with some other sub-
stance. The crystalline coloring-matter, the haemoglobin or oxyhaemo-
globin, which may be isolated from the blood, is considered, according
to Hoppe-Seyler, as a cleavage product of this compound, but it acts
in many ways unlike the questionable compound itself. This compound
is insoluble in water and uncrystallizable. It strongly decomposes
hydrogen peroxide without being oxidized itself; it shows a greater resist-
ance to certain chemical reagents (as potassium ferricyanide) than the
free coloring-matter; and, lastly, it gives off its loosely combined oxygen
much more easily in vacuum than the free pigment. To distinguish
between the cleavage products, the haemoglobin, and the oxyhaemoglobin,
Hoppe-Seyler calls the compound of the blood-coloring matter of the
venous blood-corpuscles phlebin, and that of the arterial arterin. Other
investigators, such as H. U. Kobert and Bohr,2
the latter calling the
pigment of the blood-corpuscles hcemochrom, are of a similar opinion.
Since the above-mentioned combinations of the blood-coloring matters
with other bodies, for example (if they really do exist) with lecithin, have
not been closely studied, the following statements will apply only to the
free pigment, the haemoglobin.
The color of the blood depends in part on haemoglobin and in part
on a molecular combination of this substance with oxygen, the oxy-
hemoglobin. We find in blood after asphyxiation almost exclusively
haemoglobin, in arterial blood disproportionately large amounts of
oxyhemoglobin, and in venous blood a mixture of both. Blood-color-
ing matters are also found in striated as well as in certain smooth muscles,
and lastly in solution in different invertebrates, although this pigment
is not quite identical with that from higher animals. The quantity of
haemoglobin in human blood may indeed be somewhat variable under
different circumstances, but amounts to about 14 per cent on an average,
or 8.5 grams for each kilo of the weight of the body.
Haemoglobin belongs to the group of compound proteins, and yields
as cleavage products, besides very small amounts of volatile fatty acids
and other bodies, chiefly a protein globin, and a coloring-matter, hcemo-
2
Hoppe-Seyler, Zeitschr. f. physiol. Chem., 13, 479; H. U. Kobert, Das Wirbeltier-
blut in mikro-kristallogr. Hinsicht, Stuttgart, 1901; Bohr, Centralbl. f. Physiol., 17,
p. 688.
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