2 PROTEINS 355 



When adenosine triphosphate (ATP) is added, F-actomyosin con- 

 tracts violently. Under the electron microscope F-actomyosin consists 

 of fine filaments and, after treatment with ATP, coarse threads. 

 However, as there is no change in the X-ray pattern, the syneresis 

 which occurs has been declared to be intermolecular and not intra- 

 molecular (Perry, Reed, Astbury and Spark, 1948). A gel of 2-3% 

 actomyosin throws out so much water by dehydration as to become 

 a dense gel of 50% protein. The mechanism of this contraction is not 

 vet thoroughly understood. 



Optics of striated muscle fibres. The safest way to assess the micro- 

 scopic structure of the highly differentiated striated muscle fibre is 

 between crossed nicols (Vles, 191 i; v. Muralt, 1933; Schmidt, 

 1937a). This circumvents many sources of error, such as the com- 

 plicated diffraction phenomena of striated sj^stems (Pfeiffer, 1942 b; 

 L.\NGELAAN, 1 946), and the changes in structure which are greatly, 

 though sometimes wrongly, feared in the fixation of tissues. 



The muscle fibre is lo to loo ^ in width and is enclosed in a thin 

 skin, the sarcolemma. It disintegrates into optically resolvable fibrillae 

 about I [x thick and at roughly 0.5 ji distance from each other. The 

 visible fibrillae consist of bundles of parallel submicroscopic ele- 

 mentary fibrils (HiJRTHLE, 1931). The sarcoplasm, which surrounds 

 the fibrils on all sides, lies in between the myofibrils. Essentially it 

 consists of muscle albumin, or myogen, while the fibrils are identical, 

 in the main, with muscle globulin, or actomyosin. The sarcoplasm is 

 always isotropic, but myofibrils are birefringent and exhibit the 

 familiar segmentation into bright, so-called Q and A bands and dark, 

 very weakly birefringent (usually called isotropic) I bands, which are 

 subdivided by a stronger birefringent Z band. There are accumu- 

 lations of nucleic acids of the adenyl nucleotide type in the semi- 

 isotropic I sections (Caspersson and Thorell, 1941). The re- 

 markable part of this structure is that all the fibrils of a muscle fibre, 

 though independent, have their bright and dark bands at exactly the 

 same level, with the result that the entire fibre is evenly striated. 



The coincidence of the strongly and weakly birefringent bands is 

 due to the division of the individual fibrils after the striation of the 

 original mother fibrils has occurred. Despite the conspicuous optical 

 differentiation, the fibrils are not transversally subdivided, but run in 

 uninterrupted succession through the entire length of the fibre. Their 



