THE NERVOUS TISSUES. 



F 'G. 87. 



ferent portions of their course. Every medullated nerve-fibre loses 

 its white substance of Schwann and becomes non -medullated before 

 reaching its ultimate distribution. The majority of nerve-fibres 

 constituting the great cerebro-spinal tract may be classed as med- 

 ullated, although numbers of gray fibres likewise occur here ; the 

 non-medullated fibres are especially numerous in the sympathetic 

 system, where they predominate, as well as in certain of the cranial 

 nerves, as the olfactory. While the character of the fibre, as to 

 whether it is motor or sensory, bears no relation to its size, the 

 length of the fibre seems to directly influence 

 its diameter, since fibres having long courses 

 possess greater width than those extending for 

 much shorter distances. 



A typical medullated nerve-fibre consists of 

 the following parts : 



1. The axis -cylinder, surrounded, possibly, 

 by its sheath, or axilemma (Kiihne). 



2. The medullary substance, or white matter 

 of Schwann. 



3. The neurilemma, or sheath of Schwann, 

 with the nerve-corpuscles. 



Perfectly fresh, uninjured, medullated 

 nerve-fibres, when examined by transmitted 

 light, appear as homogeneous, hyaline cylinders, 

 with dark contours and no appreciable structure ; 

 seen by reflected light, the fatty character of the 

 medullary substance is indicated by the glisten- 

 ing appearance of the fibres, and their dull white 

 color when viewed in masses. Shortly after 

 death the fibres exhibit characteristic double 

 contours, enclosing an apparently structureless 

 centre ; later, the fibres become mottled by 

 irregular spherical masses, derived from the dis- 

 torted medullary substance. 



The axis -cylinder appears, in fresh nerves or 

 in those fixed with osmic acid and teased, as an 

 inconspicuous, clear, delicate rod extending along the central part 

 of the fibre, or, perhaps, projecting beyond the outer sheaths at 

 the broken end. The longitudinal striations occasionally seen, 

 under high amplification, in carefully fixed preparations, are indi- 

 cations of the ultimate fibrillse of which the axis-cylinder is com- 

 posed ; these fibrillae are cemented together by a finely granular, 

 interstitial substance, or neuroplasm (Kolliker). According to 

 Kuhne, the axis-cylinder is enveloped by a special, delicate, elastic 



Nerve-cell from a sym- 

 pathetic ganglion of frog, 

 showing the tortuous course 

 and terminal net-work of 

 the spiral fibre : , neuri- 

 lemma continued as a deli- 

 cate sheath. (After J\et- 

 zius.) 



