The =pericardium= is attached to the sternal part of the diaphragm by a
fibrous band, and is connected with the sternum only by the mediastinal
pleura.
The =heart= is almost globular in form, the apex being much blunter than
in the other animals, and marked by a notch. Its long axis is almost
parallel with the sternum and the apex (covered by the pericardium) is
directed against the sternal part of the diaphragm. Its weight averages
about 0.8 to 1 per cent. of the body-weight.[185]
The base extends forward to a transverse plane through the lower ends
of the third pair of ribs. The apex lies opposite the sixth costal
cartilage in expiration. On the right side the pericardium is in
direct contact with the chest-wall over a narrow triangular area (base
anterior) from the fourth to the sixth intercostal space inclusive. On
the left side the area of contact is smaller and is at the fourth,
fifth, and sixth cartilages.
The two coronary arteries may arise by a common trunk. Each divides into
circumflex and descending branches. The left artery is twice as large as
the right one.
The great coronary vein ascends in the left longitudinal groove to the
coronary groove, in which it winds around the posterior face of the
heart to the right side and opens into the right atrium below the
posterior vena cava. Near its termination it receives one or two veins
which ascend along the right longitudinal furrow. Several small veins
from the wall of the right ventricle open into the right atrium
separately or by a common trunk.