Bird ventricular myocytes are long, thin and lack t-tubules, like those of fish, amphibians and reptiles. However, bird hearts achieve higher contractile rates and are capable of stronger pressure development than many mammalian and all ectothermic species. Excitation-contraction (E-C) coupling in vertebrate hearts is underpinned by calcium diffusion between the calcium release units (CRUs), formed by clusters of ryanodine receptors present on the surface of the sarcoplasmic reticulum (SR). In birds, calcium release at peripheral couplings (PCs) must diffuse to internal corbular SR (cSR) structures for propagation of the calcium signal. E-C coupling may be limited when distances between CRUs are greater than that which Ca2+ can diffuse across. These distances are therefore crucial in determining whether further calcium-induced calcium release (CICR) occurs deep inside these cells despite the absence of t-tubules.This project used electron tomography (ET), a 3D microscopy technique, to characterise the SR network, in all four chambers of the heart of White Leghorn chickens. Nearest edge-edge distances between PCs and cSR, and geometric data about individual cSR were obtained. Reconstruction with ET demonstrates the pathways for calcium diffusion throughout the cell. ET revealed a large difference in the distances between cSR compared to that reported previously, possibly due to the methodology of data collection. This thorough 3D structural characterisation will be valuable when functional work is done to study the underlying calcium transient in the cardiomyocytes of this animal.