HISTOLOGY OF NERVES 85 



Tease in glycerine a small piece of nerve that has been treated 

 with osmic acid : examine with the high power a single nerve 

 fibre : note the following points : 



i. The medullary sheath is stained darkly in conse- 

 quence of its fatty nature : it is interrupted at 

 intervals by the nodes of Ranvier. 

 ii. The nodes of Ranvier are spots where the medul- 

 lary sheath is absent, and the primitive sheath 

 forms constrictions touching the axis cylinder. 

 iii. The axis cylinder is the central cylindrical rod, 

 the essential part of the nerve fibre : it is clearly 

 visible at the nodes, and is much less deeply 

 stained than the medullary sheath, 

 iv. Nuclei are seen projecting into the medullary sheath 



about midway between the nodes. 



Tease a small piece of fresh nerve in chloroform : this will 

 partially dissolve the fatty medullary sheath and so render t/te 

 primitive sheath and axis cylinder more clearly visible : note 

 i. The primitive sheath, or sheath of Schwann. 

 ii. The axis cylinder. 



b. Non-medullated nerve fibres occur chiefly in the sympa- 

 thetic nerves : they branch and anastomose ; and they 

 have no medullary sheath. 



2. Nerve cells : Tease in glycerine a small fragment of the 

 ventral coi'nu of the spinal cord of the ox (lumbar region) : cover, 

 and examine with low and high powers : note 



i. The nerve cells : large nucleated cells with many 



branching processes. 

 ii. The nerve fibres. 



iii. The fine connective tissue binding the several parts 

 together. 



3. Structure of the Spinal Cord. 



Take one of the prepared transverse sections of spinal cord of 

 frog ; mount in balsam, and examine with low and high powers : 

 note the following points : 

 a. With the low power. 



i. The section is bilaterally symmetrical, and oval in 

 shape ; the transverse diameter considerably ex- 

 ceeding the vertical. 



