cope. Numerous white
corpuscles of different kinds and sizes will be found. Make sketches.

*To prepare Models of Red Corpuscles.*--Several models of red corpuscles
should be prepared for the use of the class. Clay and putty may be pressed
into the form of red corpuscles and allowed to harden, and small models
may be cut out of blackboard crayon. Excellent models can be molded from
plaster of Paris as follows: Coat the inside of the lid of a baking powder
can with oil or vaseline and fill it even full of a thick mixture of
plaster of Paris and water. After the plaster has set, remove it from the
lid and with a pocket-knife round off the edges and hollow out the sides
until the general form of the corpuscle is obtained. The models may be
colored red if it is desired to match the color as well as the form of the
corpuscle.

CHAPTER V - THE CIRCULATION

A Carrier must move. To enable the blood to carry food and oxygen _to_ the
cells and waste materials _from_ the cells, and also to distribute heat,
it is necessary to keep it moving, or circulating, in all parts of the
body. So closely related to the welfare of the body is the circulation(17)
of the blood, that its stoppage for only a brief interval of time results
in death.

*Discovery of the Circulation.*--The discovery of the circulation of the
blood was made about 1616 by an English physician named Harvey. In 1619 he
announced it in his public lectures and in 1628 he published a treatise in
Latin on the circulation. The chief arguments advanced in support of his
views were the presence of valves in the heart and veins, the continuous
movement of the blood in the same direction through the blood vessels, and
the fact that the blood comes from a cut artery in jets, or spurts, that
correspond to the contractions of the heart.

No other single discovery with reference to the human body has proved of
such great importance. A knowledge of the nature and purpose of the
circulation was the necessary first step in understanding the plan of the
body and the method of maintaining life, and physiology as a science dates
from the time of Harvey's discovery.

*Organs of Circulation.*--The organs of circulation, or blood vessels, are
of four kinds, named the heart, the arteries, the capillaries, and the
veins. They serve as contrivances both for holding the blood and for
keeping it in motion through the body. The heart, which is the chief organ
for propelling the blood,