VOL. 12 (1953) SERIES ELASTIC COMPONENT IN MUSCLE I33 



degrees of extension which can be produced by stretching a contracted muscle. In 

 such pictures the M membranes and the two adjoining narrower membrane-hke densities 

 are missing, suggesting that these formations, easily recognizable in resting muscle, 

 consisted of the fully coiled elastic matter. 



The chemical nature of the elastic material forming the elastic segment of the 

 protofibrils is unknown. It is not impossible that the building material is Bailey's 

 tropomyosin, which has no contractility. Sections in which the protofibrils absorbed the 

 electron stain to a marked degree suggest that the protofibrils, in their elastic segment, 

 have a spiral structure with a periodicity of 80 A. 



The importance of the elastic component in series for the physiology and pathology 

 of muscle needs no emphasis. Its physiological implications have clearly been recognized 

 by Levin and Wyman while the pathological importance, especially for heart dilation, 

 has been emphasized by Reichel. 



Hill has set the limit of the elastic extensibihty of the parallel elastic component 

 to 10-15%. Our pictures suggest a wider limit. They also explain why the resting muscle 

 3hows a normal thermo-elastic behaviour on extension and why this behaviour never 

 yielded a clue to the understanding of contraction ; extensibility and contractility being 

 the qualities of different structures. 



SUMMARY 



Electron microscopic pictures of the muscle show that the protofibrils of the cross striated muscle 

 are built alternatingly of contractile and elastic segments. The division lies in histological dimensions. 

 In its middle everj^ sarcomer contains such an elastic segment. 



RESUMfi 



La microscopie electronique du muscle strie montre que les protofibrilles sont formes de seg- 

 ments alternativement contractiles et elastiques. La division est situee dans des dimensions histolo- 

 giques. II y a un segment elastique au milieu de chaque sarcomere. 



ZUSAMMENFASSUNG 



Aufnahmen des Muskels mit den Elektronenmikroskop zeigen, dass die Protofibrillen des quer- 

 gestreifen Muskels abwechselnd aus kontraktilen und elastischen Segmenten aufgebaut sind. Die Un- 

 terteilung liegt in histologischen Dimensionen. In der Mitte jedes Sarkomers befindet sich ein elasti- 

 sches Segment. 



REFERENCES 



K. Bailey, Nature, 157 (1946) 368. 



A. V. Hill, Proc. Roy. Soc. B. 137 (1950) 273: 130 (1951 325; ^39 (1952) 464- 

 H. Latta and J. F. Hartman, Proc. Soc. Exptl. Biol. Med., 74 (1950) 436. 

 A. Levin and J. Wyman, Proc. Roy. Soc. B, loi (1927) 218. 



S. B. Newman, E. Borysko and M. Swerdlow, /. Res. Natl. Bureau Stand., 43 (1949) 183. 

 H. Reichel, Verhandl. deut. Ges. Kreislaufforschg, 16 (1950) 13. 



H. Reichel, Verhandl. deut. Ges. Kreislaufforschg, 16 (1950) 13; Z. Biol., 104 (1951) 4^9; 105 (1952) 

 7. 73. 162. 



Received April 3rd, 1953 



