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1. Structural analysis of voltage clamp preparations of sheep cardiac Purkinje fibres was carried out using methods based on light and electron microscopic observations. Results demonstrate the marked structural variability in preparations that appear, outwardly, simple.

2. The length of the cell aggregate was measured in vitro, and the mean area of cross-section, by light microscopic methods in serially sampled transverse sections. The number of intercellular clefts and path lengths of cell profiles distributed at the lateral surface and along clefts were determined from photomicrographs. Accuracy of these estimates was improved by obtaining, electron microscopically, values representing the degree of membrane folding in longitudinal and transverse planes.

3. Cleft width was evaluated from electron micrographs. Measurements made on sections that there tilted through wide arcs with a goniometer indicate that cleft width is quite variable and, on average, somewhat greater than 400 Å.

4. A three-dimensional core-conductor model is presented to aid in quantitative interpretation of electrophysiological experiments. Application of the model to individual preparations requires evaluation of the length and perimeter of cross-section of cell aggregates and of the mean number, width and depth of intercellular clefts, with appropriate corrections for fine sarcolemmal folding. Methods are given for estimating these structural parameter values from measurements of external dimensions of cell aggregates.

5. The core-conductor model is applied in an accompanying analysis of the linear electrical characteristics of Purkinje membrane.