HISTOLOGY OF NERVES. 95 



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 the 

 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 fragmerit of the 

 ventral cornu of the spinal cord of the ox (lumbar region) : cover , 

 ■and examine with low and high jjowers : 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 p)oints. 



a. With the low power. 



i. The section is bilaterally symmetrical, and oval in 

 shape; the transverse diameter considerably ex- 

 ceeding the vertical. 



ii. The ventral or " anterior" fissure is a broad and 

 shallow median cleft : there is in the frog no 

 distinct dorsal fissure. 



iii. The white matter forms the outer part of the cord, 

 and is chiefly composed of medullated nerve fibres. 



iv. The grey matter forms the central part of the cord, 

 and is composed of a dense network of non- 

 medullated nerve fibres, in which are imbedded 

 numerous nerve cells. 



