Comparative anatomy: in praise of a powerful approach to elucidate mechanisms translating cardiac excitation into purposeful contraction.

This review wishes to illustrate and, thus, reemphasize the importance of descriptive comparative anatomy for the elucidation of mechanisms driving cardiac function at different levels of spatial resolution. The following examples have been chosen: 1. the cardiac conduction system; and 2. the sarcoplasmic reticulum of cardiac and skeletal muscle. Both examples demonstrate that anatomy and geometry dictate the mechanistic behaviour of the systems under discussion, and that precise knowledge of the architecture of biological systems, in general, is crucial for an understanding of how function is consummated. The detailed comparative display and discussion of the sarcoplasmic reticulum's architecture serves the additional purpose of exposing important anatomical and geometric features of this organelle at a time when considerable efforts are being expended toward the unravelling of the mechanism of excitation-contraction coupling; anatomy is manifestly critical to these efforts.