Stcreological techniques of point and intersection counting were used to measure morphological parameters from light and electron mlcrographs of frog skeletal muscle. Re.sults for sartoHus muscle arc as follows: myofibrils comprise 83 % of fiber volume; their surface to volume ratio is 3.8/~m -I. Mitochondria comprise 1.6 % of fiber volume. Transverse tubules comprise 0.32 % of fiber volume, and their surface area per volume of fiber is 0.22/~m -a. Terminal cistcrnac of the sarcoplasmic rcticuium comprise 4. 1% of fiber volume; their surface area per volume of fiber is 0.54/~m -I. Longitudinal sarcoplasmic rcticulum comprises 5.0 % of fiber volume, and its surface area per volume of fiber is 1.48 #m -I. Longitudinal bridges between terminal cistcrnac on either side of a Z disk wcrc observed infrequently; they make up only 0.035 % of fiber volume and their surface area per volume of fiber is 0.009/~m -t. T-SR junction occurs over 67 % of the surface of transverse tubules and over 27 % of the surface of terminal cistcrnac. The surface to volume ratio of the cavcolac is 48 /~m-~; cavcolac may increase the sarcolcmmal surface area by 47 %. Essentially the same results wcrc obtained from scmitcndinosus fibers.

SUMMARY1. Measurements combining the techniques of point counting and line integration were performed on light and electron micrographs of Purkinje fibres from the sheep's heart. The measurements were aimed at determining membrane areas of importance for the cellular electrophysiology of this tissue.2. The mean volume fractions of the cells occupied by various constituents were: myofibrils, 0-234; mitochondria, 0-103; and nuclei, 0-009. The mean volume fraction of the fibres occupied by the interspaces between the tightly packed cells was 0-0023.3. The mean fractions ofintercellular surface area occupied by junctional specializations were: nexus, 0-17; desmosome, 0-023; and fascia adherens, 0-014.4. The mean surface to volume ratio of the Purkinje cells and fibres was 0-46 4ull which is 11-5 times the value of the surface to volume ratio of a long right circular cylinder 100 ice in diameter.5. There are two reasons for the increment in the surface to volume ratio of the fibre (when compared to that of a long right circular cylinder 100 / t t in diameter): the multicellular composition of the fibres and the extensive folding of the surface of the cells. 6. After correction for the intercellular nexal area the surface to volume ratio of a long cylindrical fibre 100 ,u in diameter was 0-39 pIt, or about 10 times the value for a long right circular cylinder 100,u in diameter. The surface to volume ratio of the tissue interspaces in the same fibre was 170#-1.7. It was concluded that the total sarcolemmal area in this tissue is great enough so that the specific membrane capacitance could be about 1 ulF/cm2 and the specific membrane resistance 20,000 Q cm2.

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