402 



GENERAL STRUCTURE AND FUNCTIONS 



solution of sodium hydrate, and afterward boiling them for an instant 

 in the same fluid. This extracts the greater part of their contents, and 

 leaves the tubular sheath in the form of an empty cylindrical canal. Jn 

 its general chemical relations, the tubular sheath is similar to the sar- 

 colemma of muscular fibre, its principal physical properties being its 

 cohesion and elasticity. Its physiological function is undoubtedly that 

 of a protecting envelope, by which the internal portions are maintained 

 in place and preserved from mechanical injury. 



The Medullary Layer. Immediately within the tubular sheath is a 

 layer of transparent, highly refractive, nearly fluid material, of oleagi- 

 nous consistency, termed the medullary layer, or medulla, which gives 

 to the nerve fibres, and the tracts composed of them, a white and 

 shining aspect. This substance is readily altered by a diminution of 

 temperature, or by the contact of unnatural fluids, even by exposure to 

 the air or the imbibition of water, or by the ordinary manipulations 

 required in preparing it for microscopic examination. Under these in- 

 fluences it undergoes a sort of coagulation, being increased in density 

 and in refractive power, so that both its external and internal limits are 

 indicated by a dark and strongly marked outline. This gives to the 

 nerve fibre the very characteristic appearance of a cylinder with a 

 double contour, presenting two distinct parallel outlines at each edge ; 

 an appearance by which it may be easily distinguished from any other 

 anatomical element. As the coagulation of the medullary layer goes 

 on, its outlines become more or less irregular, and after a certain time 

 it involves the whole of the fibre in a more or less confused mass of 

 irregularly refracting substance. The fibres containing a medullary 

 layer, and exhibiting the characteristic double contour due to its pre- 

 sence, are called " medullated nerve fibres." 



In the smaller variety of nerve fibres from the substance of the brain 



and spinal cord, the external 

 tubular sheath is wanting, or 

 at least cannot be demon- 

 strated ; and such fibres, owing 

 to their want of support and 

 their soft consistency, are read- 

 ily distorted by accidental pres- 

 sure, or by the contact of rea- 

 gents. They become swollen 

 or varicose at many points; 

 and the medullary substance 

 is forced out or exudes from 

 their torn extremities in irre- 

 gularly globular, fusiform, or 

 filamentous masses, which show 

 on their exterior the double 



NERVE FIBRES, from the white substance of contour due to a Superficial 

 the brain. a, a, a Portions of the myeline, pressed , , , 



out, and floating in irregularly rounded drops. Coagulation. 1 hese detached 



Fig. 135. 



