356 



FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES 



III 



jS-form, but the folded a condition that is found. True, the modifi- 

 cation of myosin to the ^-form can also be forced upon the muscle 

 by artificial extension (Astbury and Dickinson, 1935 b), but the 

 a-form always occurs in the natural state. Hence it must be assumed 

 that the polypeptide molecules in the relaxed muscle run, as in un- 

 stretched hairs, in folded chains parallel to the fibre through the 

 fibrillae. This is where the X-ray method is at a distinct disadvantage 

 as compared with polarization optics, for it fails to distinguish the 

 more strongly birefringent Q sections of striped muscles from the 

 almost isotropic I bands. 



Fig. 177. Electron micrographs of striated muscle fibres (from Hall, Jakus and Schmitt 

 1946). Above: relaxed; below: contracted. 



