358 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



whether myosin could be a part of the incrusting material. This does 

 not seem likely, since the ratio of myosin to actin is 2.5 (or even 3) 

 to I (Snellman and Erdos, 1949) so that myosin cannot be an ac- 

 cessory substance in the muscle fibre, but must be incorporated in 

 the fibrillar material. It is likely that potassium ions are part of the 

 dense substance of the Q bands, which is rich in ash, as disclosed by 

 microincineration. All incrusting substances can be removed by 

 washing without disturbing the course of the microfibrils, whereupon 

 a perfectly smooth myofibril results. 



Present information on the fine-structure of myofibrils is detailed 

 and extensive, but still confusing. Matoltsy and Gerendas (1947) 

 claim to have found an optically negative N-substance incrusting the 

 I segments, whereas this segment is free from interfibrillar material 

 according to Rgzsa, Szent-Gyorgyi and Wyckoff (1950), so that 

 its semi-isotropy is difficult to understand. Further, on the ground of 

 the negative fluctuation of the birefringence during contraction, it is 

 generally accepted that the Q segments shorten more than the I 

 segments. Hall, Jakus and Schmitt (1946), on the contrary, have 

 observed in the electron microscope that the Q band of contracted 

 myofibrils does not change, whereas the I band is shortened con- 

 siderably, accounting for almost the whole contraction, which 

 amounts to 40% of a sarcomere (relaxed about 2 /z, contracted 1.2 fj). 



By staining with phosphotungstic acid. Hall, Jakus and Schmitt 

 (1945) were able to detect a submicroscopic banding in smooth 

 muscle, the fibre period being 725 A. It would therefore seem that 

 the banding of protein fibrils is a common property, resulting, as the 

 electron microscope discloses, from the periodic dense and loose 

 packing of protein or phosphorous substances. 



The mechanism of muscular contraction. There are several ways of at- 

 tacking the important problem of muscular contraction: thermo- 

 dynamic, chemical and morphological views may help to find a 

 consistent explanation. The thermodynamic approach has tried to 

 make the disorientation of molecular elements responsible for the 

 liberation of energy when the fibre contracts (cf. Bailey, 1942). Bio- 

 chemical investigations show, however, that the energy is liberated 

 by the reaction of myosin and adenosine triphosphate, this nucleotide 

 being dephosphorylated and the liberated phosphoric acid used for 

 the phosphorolysis (see p. 314) of glycogen. The enzyme adenosine 



