216 STUDENTS HISTOLOGY 



THE NERVOUS SYSTEM 



Structural Elements 



The elements of the nervous system are : 



1. Nerve -Fibers. 



2. Nerve -Cells. 



3. Connective Tissue and Neuroglia. 



4. Peripheral Termini. 



NERVE-FIBERS 



Nerve -fibers are of two sorts, medullated or white, and non- 

 medullated or gray. 



A typical medullated nerve-fiber consists of three portions; viz., 

 a central conducting portion, the axis -cylinder; the medullary 

 sheath, or white substance of Schwann; and the enveloping connec- 

 tive tissue substance, the neurilemma. Such fibers are found 

 largely in the trunks of the cerebro- spinal system, while medul- 

 lated fibers devoid of the neurilemma exist in the optic and 

 acoustic nerves, the spinal cord, and the brain. 



The axis -cylinder may be seen to be split up longitudinally, 

 and is found to be composed of fine primitive or ultimate fibrillae, 

 which may present minute varicosities or swellings at irregular 

 intervals. 



The white substance of Schwann, which is largely fatty, pre- 

 sents under the microscope the most prominent feature of medul- 

 lated nerves, affording a nearly complete investment of the nerve- 

 axis. 



The neurilemma is an elastic envelope, which completely 

 invests the medullary substance. This tubular membrane is 

 nucleated, and at intervals is constricted so as to reach the axis- 

 cylinder. These constrictions are called the nodes of Ranvier. 

 The neurilemma presents a single nucleus, with a small amount of 

 protoplasm between each two of these nodal points. The con- 

 strictions do not, however, affect the even caliber or continuity 

 of the axis -cylinder. 



The distance between two nodes of Ranvier may be as much as 

 a millimeter, or it may be less than that. 



Non- medullated nerve -fibers are found chiefly in the sympa- 



