, _i.e._, proteids, carbohydrates, fats, and salts (Chapter IX).
Three kinds of proteids are found in the plasma, called _serum albumin_,
_serum globulin_, and _fibrinogen_. These resemble, in a general way, the
white of raw egg, but differ from each other in the readiness with which
they coagulate. Fibrinogen coagulates more readily than the others and is
the only one that changes in the ordinary coagulation of the blood. The
others remain dissolved during this process, but are coagulated by
chemical agents and by heat. While all of the proteids probably serve as
food for the cells, the fibrinogen, in addition, is a necessary factor in
the coagulation of the blood (page 31).

The only representative of the carbohydrates in the plasma is _dextrose_.
This is a variety of sugar, being derived from starch and the different
sugars that are eaten. The _fat_ in the plasma is in minute quantities and
appears as fine droplets--the form in which it is found in milk. While
several mineral salts are present in small quantities in the plasma,
_sodium chloride_, or common salt, is the only one found in any
considerable amount. The mineral salts serve various purposes, one of
which is to cause the proteids to dissolve in the plasma.

2. _The wastes_ are formed at the cells, whence they are passed by the
lymph into the blood plasma. They are carried by the blood until removed
by the organs of excretion. The two waste products found in greatest
abundance in the plasma are carbon dioxide and urea.

The substances dissolved in the plasma form about 10 per cent of the whole
amount. The remaining 90 per cent is water. Practically all the
constituents of the plasma, except the wastes, enter the blood from the
digestive organs.

*Purposes of Water in the Blood.*--Not only is water the most abundant
constituent of the blood; it is, in some respects, the most important. It
is the liquefying portion of the blood, holding in solution the
constituents of the plasma and floating the corpuscles. Deprived of its
water, the blood becomes a solid substance. Through the movements of the
blood the water also serves the purpose of a transporting agent in the
body. The cells in all parts of the body require water and this is
supplied to them from the blood. Water is present in the corpuscles as
well as in the plasma and forms about 80 per cent of the entire volume of
the blood.

*Coagulation of the Blood.*--If the blood is exposed to some unnatural
c