coagulation.
These properties are all accounted for through the different materials
that enter into the formation of the blood.

[Fig. 8]

Fig. 8--Blood corpuscles, highly magnified. _A._ Red corpuscles as they
appear in diluted blood. _B._ Arrangement of red corpuscles in rows
between which are white corpuscles, as may be seen in undiluted blood.
_C._ Red corpuscles much enlarged to show the form.

*Composition of the Blood.*--To the naked eye the blood appears as a thick
but simple liquid; but when examined with a compound microscope, it is
seen to be complex in nature, consisting of at least two distinct
portions. One of these is a clear, transparent liquid; while the other is
made up of many small, round bodies that float in the liquid. The liquid
portion of the blood is called the _plasma_; the small bodies are known as
_corpuscles_. Two varieties of corpuscles are described--the _red_
corpuscles and the _white_ corpuscles (Fig. 8). Other round particles,
smaller than the corpuscles, may also be seen under favorable conditions.
These latter are known as _blood platelets_.

*Red Corpuscles.*--The red corpuscles are classed as cells, although, as
found in the blood of man and the other mammals (Fig. 9), they have no
nuclei.(6) Each one consists of a little mass of protoplasm, called the
_stroma_, which contains a substance having a red color, known as
_hemoglobin_. The shape of the red corpuscle is that of a circular disk
with concave sides. It has a width of about 1/3200 of an inch (7.9
microns(7)) and a thickness of about 1/13000 of an inch (1.9 microns). The
red corpuscles are exceedingly numerous, there being as many as five
millions in a small drop (one cubic millimeter) of healthy blood. But the
number varies somewhat and is greatly diminished during certain forms of
disease.

[Fig. 9]

Fig. 9--Red corpuscles from various animals. Those from mammals are without
nuclei, while those from birds and cold-blooded animals have nuclei.

It is the _function_ of the red corpuscles to serve as _oxygen carriers_
for the cells. They take up oxygen at the lungs and release it at the
cells in the different tissues.(8) The performance of this function
depends upon the hemoglobin.

*Hemoglobin.*--This substance has the remarkable property of forming, under
certain conditions, a weak chemical union with oxygen and, when the