carbamate (0.2 to 0.6%), urea (0.02 to 0.05%),
creatine, creatinine, uric acid, xanthine, hypoxanthine and occasionally
hippuric acid. Three ferments are also described as being present: (1) a
glycolytic ferment exerting an action upon dextrose; (2) a lipase or
fat-splitting ferment; and (3) a diastase capable of converting starch
into sugar.

_Salts._--The saline constituents of plasma comprise chlorides,
phosphates, carbonates and possibly sulphates, of sodium, potassium,
calcium and magnesium. The most abundant metal is sodium and the most
abundant acid is hydrochloric. These two are present in sufficient
amount to form about 0.65% of sodium chloride. The phosphate is present
to about 0.02%. Sulphuric acid is always present if the blood has been
calcined for the purposes of the analysis, and may then be present to
about 0.013%. This is, however, probably produced during the destruction
of the protein, since it has been shown that no sulphate can be removed
from normal plasma by dialysis. The amount of potassium present (0.03%)
is less than one-tenth of that of the sodium, and the quantities of
calcium and magnesium are even less.

_Formed Elements._--When viewed under the microscope the main number of
these are seen to be small yellow bodies of very uniform size, size and
shape varying, however, in different animals. When observed in bulk they
have a red colour, their presence in fact giving the typical colour to
blood. These are the _red blood corpuscles_ or _erythrocytes_ (Gr.
[Greek: erythros], red). Mingled with them in the blood are a smaller
number of corpuscles which possess no colour and have therefore been
called _white blood corpuscles_ or _leucocytes_ (Gr. [Greek: leukos],
white). Lastly, there are present a large number of small lens-shaped
structures, less in number than the red corpuscles, and much more
difficult to distinguish. These are known as _blood platelets_.

_Red Corpuscles._--These are present in very large numbers and, under
normal conditions, all possess exactly the same appearance. With rare
exceptions their shape is that of a biconcave disk with bevelled edges,
the size varying somewhat in different animals, as is seen in the
following table which gives their diameters:--

Man 0.0075 mm.
Dog 0.0073 mm.
Rabbit 0.0069 mm.
Cat 0.0065 mm.
Goat 0.0041 mm.

The coloured corpuscles of amphibia as well as of nearly all vertebrates
below mammals are bicon