Polarization microscopy has also been used to investigate the struc- 

 ture of nerve fibers. In this case, the myelin sheath shows marked bire- 

 frigence which is negative with respect to the nerve fiber axis. The axon, 

 on the other hand, displays only a weak positive birefrigence relative to 

 the fiber axis. Birefrigence studies have shown that a characteristic 

 feature of striated muscle fibers is their lonRitudinal differentiation into 



a ^ d < 

 o ^ o e 



Ck (^ a a ct 



^v 



A B 



Figure 11-26. Schematic Representation of Composite Structures Made 

 Up of Either Oriented Rods (A), or Flat Discs Stacked One Upon Another 

 (B), Which are Most Likely to Exhibit Form Birefrigence. (From Oster, G., 

 1955. "Birefrigence and Dichroism," //; Oster, G. and Pollister, A. W. (Eds.), 

 "Physical Techniques in Biological Research," 1, Academic Press, New 

 York, N. Y., Fig. 2, p. 446.) 



anisotropic and isotropic regions, and that the birefrigence of the aniso- 

 tropic region is positive with respect to the direction of the muscle fiber. 

 The specific birefrigence of chloroplasts has been demonstrated to be 

 limited to the grana, suggesting that these structures are composed of 

 parallel lamellae or discs. Later studies of chloroplast structure, using 

 the electron microscope, have confirmed the lamellated structure of 

 grana. 



248 / CHAPTER 11 



