vex and elliptical. The following are the
dimensions of some of the more common:--

Pigeon 0.0147 mm. long by 0.0065 mm. wide.
Frog 0.0223 " " 0.0157 " "
Newt 0.0293 " " 0.0195 " "
Proteus 0.0580 " " 0.0350 " "
Amphiuma 0.0770 " " 0.0460 " "

Their number also varies as follows:--

Man 4,000,000 to 5,000,000 per cub. mm.
Goat 9,000,000 to 10,000,000 " "
Sheep 13,000,000 to 14,000,000 " "
Birds 1,000,000 to 4,000,000 " "
Fish 250,000 to 2,000,000 " "
Frog 500,000 per cub. mm.
Proteus 36,000 " "

In mammals they are apparently homogeneous in structure, have no
nucleus, but possess a thin envelope. Their specific gravity is
distinctly higher than that of the plasma (1.088), so that if clotting
has been prevented, blood on standing yields a large deposit which may
form as much as half the total volume of the blood.

_Chemical Composition._--On destruction the red corpuscles yield two
chief proteins, haemoglobin and a nucleo-protein, and a number of other
substances similar to those usually obtained on the break-down of any
cellular tissue, such for instance as lecithin, cholesterin and
inorganic salts. The most important protein is the haemoglobin. To it
the corpuscle owes its distinctive property of acting as an oxygen
carrier, for it possesses the power of combining chemically with oxygen
and of yielding up that same oxygen whenever there is a decrease in the
concentration of the oxygen in the solvent. Thus in a given solution of
haemoglobin the amount of it which is combined with oxygen depends
absolutely on the oxygen concentration. The greatest dissociation of
oxyhaemoglobin occurs as the oxygen tension falls from about 40 to 20
mm. of mercury. That the oxygen forms a definite compound with the
haemoglobin is proved by the fact that haemoglobin thoroughly saturated
with oxygen (oxyhaemoglobin) has a definite absorption spectrum showing
two bands between the D and E lines, whilst haemoglobin from which the
oxygen has been completely removed only gives one band between those
lines. In association with this, oxyhaemoglobin has a typical bright red
colour, whereas haemoglobin is dark purple. A further striking
characteristic of haemoglobin is that it contains iron in its molecule.
The amount present, though small bears a perfectly definite quant