This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
It is possible by methods known to physiologists to open the chest of a living animal, such as a rabbit, made insensible by chloroform, and see its heart at work, alternately contracting and diminishing the cavities within it, and relaxing and expanding them. It is then observed that each beat commences at the mouths of the veins which open into the auricles; and from there runs over the rest of the auricles, and then over the ventricles; the auricles beginning to dilate the moment the ventricles start their contraction. Having finished their contraction, the ventricles begin to dilate, and then for some time neither they nor the auricles are contracting, but the whole heart is expanding. The contraction of any part of the heart is known as its sys'to-le, and the relaxation as its di-as'to-le, and since the two sides of the heart work synchronously, the auricles together and the ventricles together, we may describe a whole "cardiac period" or "heart-beat" as made up successively of auricular systole, ventricular systole, and pause. In the pause the heart, if taken between the finger and thumb feels soft and flabby, but during the systole it, especially in its ventricular portion, becomes hard and rigid, and diminished in size so as to force blood out of it.
What is seen when the beating heart of a living animal is exposed? When do the auricles begin to dilate? What is the state of the heart for a short time after the end of a ventricular contraction? What is meant by the systole of a part of the heart? What by the diastole? Of what does a cardiac period consist? How does the heart feel to the touch during the pause?