208 Life and Health 



each of which is a minute protoplasmic mass. Between 

 each pair of nuclei the sheath is interrupted. This point 

 is known as the node of Ranvier (Figs. 1 13 and 1 14). 



Experiment 92. Obtain a small piece of nerve from the market. 

 Place the least bit of it on a glass slide. Tease it out lengthwise 

 with needles. Note the delicate thread-like fibers. Examine with 

 the high power of the microscope. 



316. White and Gray Nerve Fibers. Some nerve fibers 

 have no inner sheath (medullary), the outer alone protect- 

 ing the axis cylinder. These 

 are known as the non-medul- 

 lated fibers. They are gray, 

 while the ordinary medullated 

 fibers are white in appearance. 

 The white nerve fibers form 

 most of the white part of the 

 brain and of the spinal cord, 



Pta I.. Nerve Cells from the and fa f fa 



Gray Matter of the Brain. . fe 



bro-spinal nerves. 



The gray fibers occur chiefly in branches from the sympa- 

 thetic ganglia, though found to some extent in the nerves 

 of the cerebro-spinal system. 



^ 317. General Structure of Nerves. The separate micro- 

 scopic threads or fibers, bound together in cords of various 

 sizes, form the nerves. Each strand or cord is surrounded 

 and protected by its own sheath of connective tissue. 

 According to its size a nerve may have one or many of 

 these strands. 



The whole nerve, not unlike a minute tendon in appear- 

 ance, is covered by a dense sheath of fibrous tissue, in 

 which the blood vessels and lymphatics are distributed to 

 the nerve fibers. 



