458 



NERVOUS SYSTEM 



, 



what elastic, homogeneous membrane, presenting oval nuclei with their 

 long diameter in the direction of the tube. This is sometimes called the 

 sheath of Schwann. It exists in all the medullated nerve-fibres, large 

 and small, except those in the white portions of the encephalon and 

 spinal cord and the trunk of the auditory nerve. It possibly exists in 

 non-medullated fibres, although its presence here has not been satisfac- 

 torily demonstrated. 



The medullary substance fills the tube and surrounds the central 

 band. This is called by various names, as myelin, white substance of 

 Schwann, medullary sheath, nervous medulla etc. It does not exist 



either at the origin of the nerves in the gray 

 substance of the nerve-centres or at the periph- 

 eral termination of the nerves, and it is not 

 a conducting element. When the nerves are 

 perfectly fresh, this substance is transparent, 

 homogeneous, and strongly refracting, like oil ; 

 but as the nerves become altered by desicca- 

 tion, the action of water, acetic acid and vari- 

 ous other reagents, it coagulates into an opaque 

 granular mass. In the white substance of the 

 encephalon and spinal cord, the neurilemma is 

 wanting and the fibres present only the axis- 

 ^. ^-Medullated nerve-fibres cylinder surrounded with the white^ substance 

 (Piersoi). of Schwann. As a post-mortem condition, these 



A, teased in salt solution; x, fibres present under the microscope varicosities 



shortly after death; y, node of , . , . . , 1-1 ^i 



Ranvier; z, post-mortem distor- at irregular intervals, which give them a pe- 

 tions of medullary substance. B, culiar and characteristic appearance. 



The medullated nerve-fibres do not have 

 substance; "' neuri " regular outlines, but present constrictions at 

 various points in their length, called the con- 

 strictions or nodes of Ranvier. At these nodes the medullary substance 

 is wanting and the neurilemma is in contact with the axis-cylinder. 



When a medullated nerve-fibre is slightly stretched, a number of 

 oblique cuts are observed running across the fibre and extending to the 

 axis-cylinder, called incisures. These involve the medullary substance 

 only and are best observed when this substance has been stained with 

 osmic acid. It is not known that they possess any physiological impor- 

 tance. 



The axis-cylinder, occupying one-fifth to one-fourth of the diameter 

 of the nerve-tube, is the conducting portion of the nerve. In the ordi- 

 nary medullated fibres, the axis-cylinder can not be seen in the natural 

 condition, because it refracts in the same manner as the medullary sub- 



an isolated stained fibre ; a, axis- 

 cylinder; r, node of Ranvier; m, 



