THE NERVOUS TISSUES. 



1003 



medullated. The medullated fibres constitute the great majority of those making 

 up the peripheral nerves and the tracts of the cerebro-spinal axis ; the component 

 fibres of the latter, however, while medullated are without the neurilemma. The 

 nonmedullated fibres, on the other hand, are chiefly prolongations (axones) from 

 the ganglion cells of the sympathetic system, although in the case of the olfactory 

 nerves the fibres are also without a myelin-coat. The dis- 

 tinction between these two classes of fibres is relative rather than 

 absolute, since every medullated nerve-fibre becomes nonmed- 

 ullated before reaching its termination, central or peripheral. 



Fig. 846. 



Nonmedullated nerve- 

 fibres in longitudinal section 

 ot splenic nerve. X 310. 



Medullated nerve-fibres vary greatly in thickness, the smallest 

 having a diameter of only .001 mm., whilst the largest may measure as 

 much as .020 mm. According to their diameter, as determined by 

 Xolliker, the medullated fibres may be grouped as fine (.002-.004 

 mm.), medium (.005-.009 mm.), and coarse (.010-.020 mm.). In 

 general, the thicker fibres are the longer and are the processes of large 

 nerve-cells ; conversely, the finer have shorter courses and belong to 

 small cells. Although subject to many exceptions, the motor fibres 

 are usually the thicker and the sensory the smaller. 



Since there are many more nerve-fibres than nerve-cells, it is evi- 

 dent that the former must undergo division along their course. Such 

 doubling always occurs at a point corresponding to a node of Ranvier, 

 never within the internodal segment, the sheaths being conlinued over 

 the two resulting fibres. On approaching their peripheral termination 

 the branching becomes more frequent and the medullary sheath thinner 

 until it ends, after which the axis-cylinder continues invested with only 

 the attenuated neurilemma. The latter, now reduced to an extremely 

 delicate covering beset with occasional nuclei, sooner or later disappears, the naked axis-cylinder 

 alone being prolonged to end finally in the varicose threads of the telodendrion. 



The nonmedullated nerve-fibres proper, also termed pale Jibres ox fibres of Reniak, include 

 those that are without the myelin sheath throughout their course. They are chiefly the axones 

 of sympathetic neurones. Devoid of medullary sheath, these fibres, often .002 mm. or less in 

 diameter, consist of only the axis-cylinder and the neurilemma, the latter being thinner and 

 more delicate than on the medullated fibres. Like the latter, the pale fibres end in telodendria 

 composed of naked axis-cylinders, bearing irregular varicosities. 



Neuroglia. — The neurones (nerve-cells and fibres) within the cerebro-spinal 

 axis are everywhere held together by a special supporting tissue known as neuroglia. 



The latter is primarily derived from the invagi- 

 nated ectoblast lining the neural tube, certain 

 elements, the spongioblasts, being devoted to the 

 production of the neuroglia, while others, the 

 neuroblasts, give rise to the neurones. At first 

 the supporting tissue is represented by greatly 

 elongated, radially disposed fibre-cells that often 

 extend the entire thickness of the wall of the 

 neural canal. Later, the neurogliar elements 

 become differentiated into (a) those bordering 

 the lumen of the canal, which are partly retained 

 as the epe7idymal cells, and {b') those which have 

 early migrated to more peripheral locations and 

 given rise to stellate cells that are converted 

 into spider-like elements, the astrocytes. Seen 

 in chrome-silver preparations (Fig. 847) these 

 appear as irregular triangular or quadrilateral 

 cells from wbose angles numerous delicate 

 fibrillae extend between the surrounding nervous 

 elements. According to Rubaschkin,^ the astro- 

 cytes are transformations from larger branched gliogenetic cells, by the conversion of 

 whose robust protoplasmic processes the delicate fibrillcz that later form the chief 



'Archivf. mikros. Anat. u. Entwick., Bd. 64, 1904. 



Fig. 847. 



Young neuroglia cells; astrocytes, from brain 

 of child. X 300. 



