Method

The body of the frog is anchored to the corkboard by inserting a metal pin through the backbone below the position of the heart. A small hook is then inserted into the muscle at the tip of the ventricle and the heart pulled upwards out of the thoracic cavity. The hook is attached, by means of a length of cotton, to a strain gauge suspended vertically above the heart.

As the heart contracts and relaxes the tension changes are detected by the strain gauge and transduced to electrical changes, which are then amplified (DC amplifier). The amplified signal is displayed on a chart recorder. Alternatively, the heart could be connected to a balanced isotonic lever and movements of the lever amplified and displayed.