360 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



values compatible with the stress measured in contracting threads of 

 polyacrylic acid when placed in o.oz N HCl. 



The matter is probably not quite as simple as this, for the charges 

 of the polypeptide thread molecules are not in the primary chain, but 

 at the extremity of the end groups of the side chains. In a later work, 

 Meyer and Picken (1937) prove by thermoelastic investigations on 

 stretched muscle fibres that, in a state of rest, the polypeptide chains 

 are mobile as in a liquid, whereas fixed bonds are established as soon 

 as the muscle is irritated ; thus the molecular framework of the muscles 

 passes from an apparently "fluid" to a solid state. It should be noted 

 that the comparison with rubber ceases to be valid under these circum- 

 stances, for in that material the polyene chains are, conversely, more 

 mobile in the contracted state and are interlocked in the elongated con- 

 dition. The interesting reaction involved in muscular induration must 

 surely take place between the end groups of neighbouring side chains. 



This is a good example demonstrating the consistency of our 

 theory of junctions. Contracted muscle fibres exhibit an extreme gel 

 structure, whereas relaxed fibres show a less tightened structure. We 

 may thus compare muscle relaxation with the transformation of the 

 plasmagel to the plasmasol in protoplasmic flow, when junctions must 

 similarly be freed to allow displacement of the structural elements. 



f. Nerves (Neurokeratin and Neuronin) 



The wjelin sheath. Myelinated nerves in Vertebrates consist of a 

 central strand enveloped in a highly birefringent sheath. The bire- 

 fringence of this sheath is produced by the embedded myelin, which 

 produces the myelin forms described on p. 54 upon the addition of 

 water. Like myelin tubes, the mvelin sheath is optically negative as 

 referred to the axial direction. Referred to the radial direction, 

 however, the birefringence is positive. Thus in a cross-section through 

 the nerves the sheath shows a positive cross, while the axoplasm 

 appears as isotropic. Since myelin comprises lecithin (Fig. 47, p. 56), 

 cephalin, cholesterol (Fig. 92, p. 138) and other anisodiametric optic- 

 ally positive molecules, they must, judging by the birefringence, be 

 orientated in the sheath with the longitudinal axis running radially. 

 Isolated myelin substances produce X-ray periods corresponding to 

 double the molecular length. There must therefore be bimolecular 

 lipid layers in the nerves. The thicknesses of the layers are given in 



