acts as a force pump, while the arteries and
veins serve as tubes for conveying the blood from place to place.
Moreover, the blood vessels are so connected that the blood moves through
them in a regular order, performing two well-defined circuits.

[Fig. 13]

Fig. 13--*Heart* in position in thoracic cavity. Dotted lines show positin
of diaphragm and of margins of lungs.

*The Heart.*--The human heart, roughly speaking, is about the size of the
clenched fist of the individual owner. It is situated very near the center
of the thoracic cavity and is almost completely surrounded by the lungs.
It is cone-shaped and is so suspended that the small end hangs downward,
forward, and a little to the left. When from excitement, or other cause,
one becomes conscious of the movements of the heart, these appear to be in
the left portion of the chest, a fact which accounts for the erroneous
impression that the heart is on the left side. The position of the heart
in the cavity of the chest is shown in Fig. 13.

*The Pericardium.*--Surrounding the heart is a protective covering, called
the pericardium. This consists of a closed membranous sac so arranged as
to form a double covering around the heart. The heart does not lie inside
of the pericardial sac, as seems at first glance to be the case, but its
relation to this space is like that of the hand to the inside of an empty
sack which is laid around it (Fig. 14). The inner layer of the pericardium
is closely attached to the heart muscle, forming for it an outside
covering. The outer layer hangs loosely around the heart and is continuous
with the inner layer at the top. The outer layer also connects at certain
places with the membranes surrounding the lungs and is attached below to
the diaphragm. Between the two layers of the pericardium is secreted a
liquid which prevents friction from the movements of the heart.

[Fig. 14]

Fig. 14--*Diagram of section of the pericardial sac*, heart removed. _A._
Place occupied by the heart. _B._ Space inside of pericardial sac. _a._
Inner layer of pericardium and outer lining of heart. _b._ Outer layer of
pericardium. _C._ Covering of lung. _D._ Diaphragm.

*Cavities of the Heart.*--The heart is a hollow, muscular organ which has
its interior divided by partitions into four distinct cavities. The main
partition extends from top to bottom and divi