THE NERVOUS TISSUES. 73 



medullary substance ; on obtaining the latter, it then courses as 

 the typical medullated fibre until near its termination, where again 

 the white substance of Schwann is wanting, the end arborizations 

 consisting of the ramifications of the naked axis- cylinder. The 

 majority of nerve-fibres constituting the great cerebro-spinal tract 

 may be classed as medullated, although numbers of gray fibres like- 

 wise 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. In other cases, as 

 conspicuously in the pneumogastric nerve of the 

 dog, distinct bundles of both medullated and FlG - 8 7- 



non-medullated fibres are associated in the for- 

 mation of a common nerve-trunk. 



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 showing the tortuous course 

 irregular spherical masses, derived from the dis- a " d term , ina } net - work . f 



*I the spiral fibre: n, neun- 



tOrted medullary Substance. lemma continued as a deli- 



The axis-cylinder appears, in fresh nerves or c ** } sheath - (After Ret ' 

 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 

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







