100 



HISTOLOGY. 



When nerve fibers are treated with a solution of silver nitrate a pre- 

 cipitate occurs at the nodes and spreads along the axis cylinder forming 

 a cross (Fig. 116, B). This has been interpreted as indicating a penetrable 

 intercellular substance at the nodes through which nutriment has access 

 to the fiber. Silver nitrate sometimes causes a transverse banding of the 

 axis cylinder, which is considered artificial and without significance. 

 In crossing the node the fibrils may spread apart forming a "biconical 

 enlargement." As shown in C, the fibrils in the midst of the enlargement 

 have been said to be thickened. The same figure suggests that the neuro- 

 lemma is not continuous with an axolemma but passes the node without 

 interruption. This is clearly shown in D, where the myelin layer also, 

 though constricted, is unbroken. The myelin has therefore been regarded 

 as an exoplasmic part of the axis cylinder. The inter-fibrillar substance 



Fat cells. 



Artery. 



Bundles of nerve fibers 



Epineurium. 



Perineurium. 



^ Endoneurium. 





FIG. 117. MEDULLATED NERVE. PART OF A CROSS SECTION OF THE HUMAN MEDIAN NERVE. X 20. 



of the nerve fiber has been said to present many characteristics of myelin. 

 The close relation of myelin to the cylinder is shown in "peripheral de- 

 generation." When a nerve is severed, that portion of the axis cylinder 

 which is cut off from the cell body from which it grew, degenerates by 

 fragmentation. The myelin at the same time breaks up into drops of 

 a different chemical nature which later disappear. The sheath cells 

 multiply. Recently it has been stated that the myelin should be considered 

 an intercellular substance due to a transformation of tissue fluid by the 

 joint activity of the axis cylinder and sheath cells. It first appears in the 

 embryo as vesicles attached to the nerve fiber. These unite to form a 

 nodular or beaded layer which later becomes smooth. The axolemma 

 is considered a condensation of the myelin framework such as occurs also 

 just beneath the neurolemma. The myelin itself is said to be derived 

 from the blood. 



