388 ANIMAL PHYSIOLOGY. 
reduction; that there are two instances in which there may be 
a single band from hemoglobin—in the one case when the solu- 
tion is very dilute and when it is very concentrated. These 
need never be mistaken for each other nor for the band of re- 
duced hemoglobin. The latter isa hazy broad band with com- 
paratively indistinct outlines, and darkest in the middle. 
It will be further noticed that in all these instances, apart 
from the bands, the spectrum is otherwise modified at each 
end, so that the darker the more centrally placed characteristic 
bands, the more is the light at the same time cut off at each 
end of the spectrum. 
If, now, to a specimen showing the two bands of oxy-hemo- 
globin distinctly a few drops of ammonium sulphide or other 
reducing agent be added, a change in the color of the solution 
will result, and the single hazy band characteristic of hemo- 
globin will appear. , 
It is not to be supposed, however, that venous blood gives 
this spectrum. Even after asphyxia it will be difficult to see 
this band, for usually some of the oxy-hemoglobin remains 
reduced; but it is worthy of note, as showing that the appear- 
ances are normal, that the blood, viewed through thin tissues 
when actually circulating, whether arterial or venous, gives 
the spectrum of oxy-hemoglobin. At the same time there can 
be no doubt that the changes in color which the blood under- 
goes in passing through the capillaries is due chiefly to loss of 
oxygen, as evidenced by the experiments before referred to; and 
the reason that the two bands are always to be seen in venous 
blood is simply that enough oxy-hemoglobin remains to give 
the two-band spectrum which prevails over that of (reduced) 
hemoglobin. We are thus led by many paths to the important 
conclusion that the red corpuscles are oxygen-carriers, and, 
though this may not be and probably is not their only func- 
tion, it is without doubt their principal one. Of their oxygen 
they are being constantly relieved by the tissues; hence the 
necessity of a circulation of the blood from a respiratory point 
of view. 
There are other gases that can replace oxygen and form 
compounds with hemoglobin; hence we have CO-hemoglobin 
and NO-hemoglobin, which in turn are replaced by oxygen with 
no little difficulty—a fact which explains why carbonic oxide is 
so fatal when respired, and, as it is a constituent of illuminat- 
ing gas, the cause of the death of those inhaling the latter is 
often not far to seek. Blood may, in fact, be saturated with 
