434 



STUDIES IN ADVANCED PHYSIOLOGY. 



away from the center from which it originated, it seems in 

 a fresh condition almost homogeneous, but when allowed to 

 stand or treated with proper re-agents it soon resolves itself 

 into three structures. 



Axis- Cylinder. Running through the 

 center of the nerve fiber as a continuation 

 of the nerve cell itself is the axis-cylin- 

 der. This is the real nerve matter of the 

 fiber, and is the only one that is really 

 physiologically concerned in the carrying 

 of nervous impulses. In fact, the two 

 other coats may sometimes be absent alto- 

 gether, but an axis-cylinder can never be 

 absent. 



Medullary Sheath. Surrounding the 

 axis-cylinder is a thick whitish-looking 

 coat called the medullary sheath. This 

 medullary sheath seems interrupted at in- 

 tervals of about one-twenty-fifth of an 

 inch, which interruptions are called the 

 nodes of Ranvier. The medullary coat 

 between two consecutive nodes contains a 

 large nucleus and gives evidence that this 

 inter-node is of cellular origin. Chem- 

 ically the medullary coat consists of a 

 substance called myelin, a substance 

 somewhat akin to that derived from cer- 

 tain connective tissues. 



Fig. 140. MEDULLATED 



NERVE-FIBER TREATED 



WITH OSMIC ACID. (After 

 Key and Retzius.) 



Primitive Sheath. Around the med- 

 ullary coat in turn is a thin epithelial coat 



E, node of Ranvier; K, 11 i ,1 1 , t 



nucleus of medullary called the primitive sheath or neurolem- 



and ax 



show. 



primitive sheath ma or by ot hers the sheath of Schwann. 



is - cylinder also 



The medullary coat ceases near the cell 

 from which the axis-cylinder arises, but the primitive sheath 

 is usually continued over the cell body itself. This sheath 

 further dips down into the nodes of Ranvier. 



