298 NERVE-TISSUE. 



(J)) Non-medullated Nerves. These nerves have the appearance 

 of being bare axis-cylinders, destitute of a myeline investment. 

 There are comparatively broad so-called Remak's fibers (1838). 

 in large numbers, in the sympathetic and in the cranial portions 

 of the olfactory and auditory nerves. These fibers exhibit on 

 their surface a number of oblong nuclei, and they have often a 

 delicate sheath, kindred to the axis-cylinder sheath of medullated 

 nerve-fibers, to which, probably, the nuclei belong. These 

 nerves are described as indistinctly fibrillar in structure, and 

 at certain intervals showing clusters of small, bead-like forma- 

 tions, giving rise to the so-called necklace appearance. All 

 nerves in the earliest stages of embryonal development are non- 

 medullated. 



Besides these broad non-medullated nerve-fibers, there are 

 others which are narrower, scattered throughout the gray sub- 

 stance of the brain and of the spinal cord. They run in bundles 

 with the medullated nerve-fibers, and also with nerves of the 

 sympathetic system. Such fibers, which are bare axis-cylinders, 

 represent the origin of all nerve-fibers, even of the medullated, in 

 the gray substance. In many instances, the medullated fibers 

 become again non-medullated upon approaching the periphery of 

 the body. 



With high amplifications of the microscope, some of the larger 

 non-medullated nerve-fibers show distinctly a delicate reticular 

 structure. Others exhibit a number of minute vacuoles in their 

 interior; still others, and these are the finest nerve-fibers, have 

 a homogeneous appearance and give no evidence of structure. 

 (See Fig. 127.) 



Many of the finest non-medullated nerve-fibers show oblong, 

 varicose enlargements along their course, and bear the name of 

 varicose nerve-fibers. The varicosities are certainly not post-mor- 

 tem appearances, as they are visible in the perfectly fresh condi- 

 tion of nerve-specimens, as stated above. 



3. TERMINATIONS OF NERVES. 



The manner in which the ultimate nerve filaments terminate 

 in the tissues and at the periphery of the body is known only in 

 part. There are two varieties, either terminations of medul- 

 lated nerves as such, or terminations of non-medullated nerves. 

 The latter are either continuations of originally medullated 



