above two of the semilunar valves. The
blood is returned to the right auricle (not to either of the venae
cavae) by the _coronary vein_.]

The longest route a portion of blood may take from the moment it leaves
the left ventricle to the moment it returns to it, is through the portal
circulation. The shortest possible route is through the substance of the
heart itself. The mean time which the blood requires to make a complete
circuit is about 23 seconds.

192. The Rhythmic Action of the Heart. To maintain a steady flow of
blood throughout the body the action of the heart must be regular and
methodical. The heart does not contract as a whole. The two auricles
contract at the same time, and this is followed at once by the contraction
of the two ventricles. While the ventricles are contracting, the auricles
begin to relax, and after the ventricles contract they also relax. Now
comes a pause, or rest, after which the auricles and ventricles contract
again in the same order as before, and their contractions are followed by
the same pause as before. These contractions and relaxations of the
various parts of the heart follow one another so regularly that the result
is called the rhythmic action of the heart.

The average number of beats of the heart, under normal conditions, is from
65 to 75 per minute. Now the time occupied from the instant the auricles
begin to contract until after the contraction of the ventricles and the
pause, is less than a second. Of this time one-fifth is occupied by the
contraction of the auricles, two-fifths by the contraction of the
ventricles, and the time during which the whole heart is at rest is
two-fifths of the period.

193. Impulse and Sounds of the Heart. The rhythmic action of the
heart is attended with various occurrences worthy of note. If the hand be
laid flat over the chest wall on the left, between the fifth and sixth
ribs, the heart will be felt beating. This movement is known as the
beat or impulse of the heart, and can be both seen and felt on
the left side. The heart-beat is unusually strong during active bodily
exertion, and under mental excitement.

The impulse of the heart is due to the striking of the lower, tense part
of the ventricles--the apex of the heart--against the chest wall at the
moment of their vigorous contraction. It is important for the physician to
know the exact place where the heart-beat should be felt, for the heart
may be displaced by disease, and