e heart are
the veins. Thus, the arteries end in the tissues in fine, hair-like
vessels, the capillaries; and the veins begin in the tissues in
exceedingly small tubes,--the capillaries. Of course, there can be no
break in the continuity between the arteries and the vein. The apparatus
of circulation is thus formed by the heart, the arteries, the
capillaries, and the veins.

184. The Heart. The heart is a pear-shaped, muscular organ
roughly estimated as about the size of the persons closed fist. It lies in
the chest behind the breastbone, and is, lodged between the lobes of the
lungs, which partly cover it. In shape the heart resembles a cone, the
base of which is directed upwards, a little backwards, and to the right
side, while the apex is pointed downwards, forwards, and to the left side.
During life, the apex of the heart beats against the chest wall in
the space between the fifth and sixth ribs, and about an inch and a half
to the left of the middle line of the body. The beating of the heart can
be readily felt, heard, and often seen moving the chest wall as it strikes
against it.

[Illustration: Fig. 69.--Diagram illustrating the Structure of a Serous
Membrane.

A, the viscus, or organ, enveloped by serous membrane;
B, layer of membrane lining cavity;
C, membrane reflected to envelop viscus;
D, outer layer of viscus, with blood-vessels at
E communicating with the general circulation.
]

The heart does not hang free in the chest, but is suspended and kept in
position to some extent by the great vessels connected with it. It is
enclosed in a bell-shaped covering called the pericardium. This is
really double, with two layers, one over another. The inner or serous
layer covers the external surface of the heart, and is reflected back upon
itself in order to form, like all membranes of this kind, a sac without an
opening.[32] The heart is thus covered by the pericardial sac, but
is not contained inside its cavity. The space between the two membranes is
filled with serous fluid. This fluid permits the heart and the pericardium
to glide upon one another with the least possible amount of friction.[33]

The heart is a hollow organ, but the cavity is divided into two parts by a
muscular partition forming a left and a right side, between which there is
no communication. These two cavities are each divided by a horizontal
partition into an upper and a lower chamber. These partitions, however,
include a set of