Nervous Systems 



785 



TABLE 76 (continued) 



VELOCITY OF CONDUCTION IN SELECTED NERVE FIBERS AS RELATED 



TO FIBER SIZE AND SHEATH THICKNESS (VELOCITY IN MOIST AIR) 



Nerve Fiber Sheath. In addition to fiber diameter, the existence of a lip- 

 oid sheath around the axon is directly correlated with velocity of conduc- 

 tion. Among vertebrates the larger fast motor and sensory fibers have mye- 

 lin sheaths u'hich blacken with osmic acid and can be readily measured, 

 whereas the slow small sympathetic fibers are said to be non-myelinated. 

 When examined in polarized light the larger fibers are seen to be positively 

 birefringent with respect to their radially directed optic axis; this is due to the 

 Hpoid sheath and the fibers are called myelotropic. The birefringence in frog 

 fibers is nearly constant among fibers 9 fiox more in diameter but diminishes in 

 smaller fibers until it is zero at 2 fx, and below that size it reverses sign.^^* 

 The birefringence of the "non-medullated" fibers, on the other hand, is 

 normally negative with respect to the radial axis, is due to proteins, and can 



