PROTEINS 



361 



Table XXXL The averay-e distance between the moIecul?r chains is 



4.8 A (BOEHM, 1933). 



Small-angle X-ray diffraction furnishes layer periods of 186 A for 

 fresh and 1 5 8 A for dried mammalian nerves. This shrinkage shows 

 that hydration water lies between the lamellae. Since a drv double 

 layer of neural myelin is only 66 A thick, it is likely that the macro- 

 period of 1 5 8 A not only includes two myelin double layers but also 

 structural protein (Schmitt, 1950b). Cf. Fig. 48, p. 57. 



TABLE XXXI 



THICKNESS OF BIMOLECULAR LAYERS OF LIPIDS IN NEURAL 



MYELIN (after BEAR, PALMER, AND SCHMITT, I941) 



Spacing in A 



Substance 



Determined by 



Calculated from 

 atomic distances 



Lecithin . . . 

 Cephalin . . . 

 Sphyngomyelin 

 Kerasin. . . . 

 Phrenosin. . . 



65 

 64 

 64 



The myelin sheath does not entirely lose its birefringence when the 

 myelin substances are extracted with fat solvents, but there then 

 appears a negative cross on the cross-section (Schmidt, 1937a, b; 

 Schmitt and Bear, 1939). This birefringence decreases appreciably 

 when the extracted cross-sections of the nerves are transferred from 

 alcohol to Canada balsam. There is therefore lamellar form bire- 

 fringence, for the radial direction remains the optical axis, just as 

 before extraction of the myelin. The submicroscopic layers must 

 consist of neurokeratin, which is to be considered as the frame 

 substance of the sheath. The polypeptide chains of this protein cannot 

 have any preferred orientation, for, if they had, there would be no 

 optical axis in the radial direction. The submicroscopic lamellae of 

 protein must therefore be fohate in texture. Schmidt (1937a, p. 306, 

 Fig. 80) assumes that there are individual submicroscopic particles of 



