. Elements of human physiology. Physiology. 240 PHYSIOLOGY the varying tension of the fibres at the beginning of the con- tractile process. Of course if the initial tension be continually increased, we finally arrive at a point at which this peripheral reactive mechanism gives way ; the heart is unable to contract against the great resistance and becomes permanently stretched and damaged. The refractory period.—At each contraction of the heart- muscle there is a sudden decomposition of contractile mate- rial which, so far at least as concerns the incidence of an external stimulus, is maximal,
. Elements of human physiology. Physiology. 240 PHYSIOLOGY the varying tension of the fibres at the beginning of the con- tractile process. Of course if the initial tension be continually increased, we finally arrive at a point at which this peripheral reactive mechanism gives way ; the heart is unable to contract against the great resistance and becomes permanently stretched and damaged. The refractory period.—At each contraction of the heart- muscle there is a sudden decomposition of contractile mate- rial which, so far at least as concerns the incidence of an external stimulus, is maximal, complete. Directly this has occurred a process of assimilation or re-formation of contractile material starts. This lasts throughout the Fm. Isometric contractions of frog's ventricle. The initial tension was continually increased from curve 1 to curve 6, each increase of tension causing a greater energy of contraction, (v. Frank.) diastolic period, and the store of contractile material is at its maximum just before the next contraction. We may in fact compare the process to a bucket, into which a stream of water is constantly flowing, and which tips up automatically and empties out its contents as soon as the water reaches a certain height. It is evident that the power of the heart- muscle to contract in response to a stimulus (its irritability) must be at a minimum immediately after the automatic dis- charge, or decomposition, has taken place, and will con- tinually increase from this point as the store of contractile material grows, until it arrives at such a height that the explosive discharge occurs spontaneously. Hence in each cardiac cycle there is a period, known as the refractory. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Starling, Ernest Henry, 1866-1927. Chicago : W. T. Keener
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