The systole of the corresponding cavities of both sides of the heart is exactly synchronous; that is to say, the two auricles contract simultaneously, and the contraction of the two ventricles follows immediately that of the auricles.

The ventricular systole follows that of the auricle so closely that no interval can be appreciated.' The rapidly succeeding acts of auricular and ventricular systole are followed by a period during which both auricles and ventricles are in diastole, which is commonly spoken of as the passive interval ox pause.

While the auricles are contracting the ventricles are relaxed, and the relaxation of the auricles commences immediately the ventricular contraction begins.

The entire cycle of the heartbeat, occupying nearly a second in the healthy adult, may be divided into three stages:-

Auricular systole. Ventricular systole. General diastole.

Curves drawn on a moving surface by three levers, which are connected with the interior of the heart, viz.

Fig. 119. Curves drawn on a moving surface by three levers, which are connected with the interior of the heart, viz.: -

Upper line shows the changes of pressure occurring in the right auricle;

Centre line shows the pressure changes within the right ventricle;

Lower line shows the changes of pressure occurring in the left ventricle.

(The smoked surface is moved from right to left.) {After Chauvian).

The exact time occupied by each phase of the cycle can only be calculated approximately. This may be done either by registering graphically the motions of the auricles and ventricles directly communicated to levers brought into contact with their surface, or by recording graphically the pressure changes which occur within the cavities, by introducing into them little elastic sacks filled with air, whence the pressure changes are communicated to an ordinary "tambour," and registered on a smoked surface.. Of the whole period of the cycle the passive interval or pause is the longest and the most variable, for in the ordinary changes in the heart's rhythm the pause alone varies. Next in duration is the ventricular systole, while the shortest is the auricular systole.

The following figures give approximately the proportion of time occupied by each part of the cycle in. the case of a horse, whose intra-cardiac tension was registered in the manner just referred to while his heart beat about fifty times in the minute: -

Proportion of cycle.

Duration in seconds.

Auricular systole

1/2

O.2"

Ventricular systole

2/6

O.4"

Passive interval

3/6

O.6"

Or if we assume the human heart to beat some seventy times a minute, each cycle would occupy about 8/10 of a second, made up as follows: -

Auricular systole

= 1/10 of a second

Ventricular systole

= 3/10 "

Pause

= 4/10 "

The duration of the auricular and ventricular systole varies little except under abnormal circumstances, but the pause is constantly undergoing slight changes. In fact, the duration of the general diastole depends upon the rate of the heart beat, being less in proportion as the heart beats more quickly.