Abstract
The tissue space of rat cardiac muscle is continuous with the subendocardial layer and with that of the epicardium. It is bounded by the sarcolemma of myocardial fibres. The spaces between the myocardial fibres are of varying dimensions. The largest areas are probably those associated with the sinoatrial and atrioventricular nodes. The extracellular material of the myocardial tissue space generally is electron-lucent. Native collagen is present as single unit fibrils or in bundles of several unit fibrils. Elastic lamellae are present only at the aortic junction. Fibroblasts are the commonest free cells in the tissue space, and 'myocardial foam cells.' Myelinated and unmyelinated nerve fibres are numerous and are found in close association with the myocardial fibres and the extensive capillary network.
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