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In the course of some experiments! to determine the influence of a rise of temperature upon the length of muscle stretched by different loads, we observed in a few cases that as the temperature rose, the ordinary heat contraction was followed by a second. This second contraction was less in extent than the first, and exerted only a slight force, for it was readily cut out, or even replaced by an elongation, when the tension was increased. In all these early experiments the muscle preparation employed was a frog’s gastrocnemius, or a combined semitendinosus and gracilis preparation, and we were at first unable to obtain the same result when using the sartorius. We accordingly determined to investigate fully the conditions under which it was to be observed. At first we directed our attention to differences in anatomical structure, which might explain the difference in behaviour of the two muscles, and of these the most notable is the presence of a large tendinous insertion, arranged in a typical manner on the surface and in the interior of the gastrocnemius. The shrinkage of tendon when a piece is dropped into boiling water is a well-known phenomenon. It is limited to a shortening of the fibres in the direction of their length, and has been studied by Engelmann and by Hermann. We soon found, however, that this could not explain the difference found, for in muscle the second contraction commences between 47° C. and 50° C., whereas the contraction of tendon does not commence until about 62° C. is reached.

IV. A study of the phenomena and causation of heat-contraction of skeletal muscle