I.] THE FROG. 137 



and a peripheral clear portion {medullary sheath); 

 the whole invested in a delicate primitive sheath, 

 obvious as a darkly stained outline. 

 /?. Non-medullated nerve fibres ; irregularly dispersed 

 among a. ; each consisting of an axis-fibre alone. 



c. The medullated nerve-fibre ; structural analysis. 



i. Tease out a bit of a fresh spinal nerve in salt 

 solution. Examine with a low power. 



a. Composed of well-defined fibres (medullated 

 nerve-fibres) mixed with white fibrous tissue. 



/?. The appearance of the nerve-fibres ; each has 

 a double contour and a highly refractive 

 border. Some fibres are regular in outline, 

 others contorted and irregular. 



ii. Examine, under a high power, selecting the least 

 contorted fibres. 



u.. The primitive sheath; look for it on bruised 

 nerve-fibres or at the ends of torn ones. 



fi. The highly refractive border (medullary sheath) 

 within the primitive sheath. 



y. The central homogeneous axis (axis fibre); 

 look for it projecting beyond the medullary 

 sheath of torn fibres. 



8. Treat with chloroform ; the medullary sheath 

 will be dissolved out, and the axis fibre 

 plainly seen. 



e. Ranvier's nodes; apparent breaks (constric- 

 tions) in the medullary sheath, seen at com- 

 paratively long intervals along the course of 

 the individual fibre. 



