Figure 2. An early demonstration of the importance of Ca²⁺ for muscle contractility.

Changes in volume were recorded from an isolated frog ventricle perfused with blood or saline solutions (more detail on the technique and Ringer's work is presented by Miller, 2004). Full contractions were only seen when Ca²⁺ was present in the perfusate. Figure reproduced with permission from Ringer (1883).