248 ELEMENTAEY BIOLOGY. [XIII. 



c. The structure of the nerve-fifoes. 



a. The delicate structureless investing mem- 

 brane (primitive sheath) ; look for it on bruised 

 nerve-fibres or at the ends of torn fibres. 



ft. The highly refractive border (medullary sheath] 

 within the primitive sheath. 



7. The central homogeneous axis (axis cylinder) ; 

 look for it projecting beyond the medullary 

 sheath of torn fibres. 



d. Treat a fresh bit of teased-out nerve with chloro- 

 form : the medullary sheath will be dissolved out, 

 and the axis cylinder plainly seen. 



g. Nerve-cells. 



1. Take a sympathetic ganglion from a recently killed 

 frog : dissect out in normal saline solution and ex- 

 amine with J obj. 



a. Among the pigment-cells, which are clustered 

 around the ganglion, will be seen numerous 

 large pale granular cells, each possessing a con- 

 spicuous clear round nucleus with a distinct 

 nucleolus. 



fi. Tease out in glycerine a Gasserian ganglion 

 from a frog's head which has been preserved in 

 Muller's fluid or chromic acid. Cells like those 

 described above will be found. 



7. Examine sections of spinal cord which have 

 been stained with carmine or hsematoxylin, 

 and note the large branched nucleated cells in 

 the grey matter especially towards the ventral 

 side of the cord. 



