The contraction of the heart, which takes place seventy to eighty times in a minute, is automatic and is due to the essential quality of the muscle which composes it.

The character, frequency and force of contraction, however, can be influenced by the nervous system and by the direct action of substances upon the heart muscle.

The heart is divided by a longitudinal partition into a right and left cavity, and these cavities are divided by transverse septa, with openings in them controlled by valves, each into two chambers termed _auricle_ and _ventricle_.

The auricle and ventricle on each side are completely separated.
The circulation of the blood through the heart is as follows: The blood, which in the veins of the body is flowing towards the heart, passes by two channels, which respectively receive the blood from the upper and lower part of the body, into the right auricle.

When this becomes distended it contracts, forcing the blood into the right ventricle; the ventricle then contracts and sends the blood into the arteries of the lungs, the passage of blood into the auricle being prevented by valves which close the opening between auricle and ventricle when the latter contracts upon its contents.