NORMAL HISTOLOGY. 



nonmedullated. The medullated fibres constitute the majority of those 

 making up the peripheral nerves and the fibre-tracts within the cerebro- 

 spinal axis. The fibres within the latter, however, while medullated are 

 without a 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 without 

 a neurilemma. The distinction between these two classes of nerve- 

 fibres, however, is relative rather than absolute, since every medullated 

 nerve-fibre becomes nonmedullated not only at its 

 origin from the cell, but also before making its 

 terminal arborization, central or peripheral. Medul- 

 lated nerve-fibres vary from 1-20 /* in thickness. 

 According to their diameter, they are grouped d&fine 

 (1-4 ju), medium (5-9 /*) and coarse (10-20 /Jt). 

 In a general way it may be said that the thicker 

 fibres are the longer and are the processes of large 

 nerve-cells; conversely, the finer fibres are shorter 

 and belong to small cells. Although subject to 

 many exceptions, the efferent (motor) fibres are 

 usually the thicker, and the afferent (sensory) the 

 thinner. 



Since there are many more peripheral nerve- 

 fibres than nerve-cells, it is evident that the former 

 must undergo division along their course. Such 

 doubling occurs always at a point corresponding to a 

 node of Ranvier, never within an internodal segment, 

 the sheaths being continued on the resulting fibres. 

 On approaching their peripheral termination, the 

 branching becomes more frequent and the medullary sheath thinner until it 

 ceases, after which the axis-cylinder continues covered by only the attenuated 

 neurilemma. The latter, now reduced to an extremely delicate covering be- 

 set with occasional nuclei, sooner or later disappears, the naked axis-cylinder 

 alone being thence prolonged to end finally in the varicose threads of the telo- 

 dendrion. The nonmedullated nerves proper, also termed the pale fibres 

 or fibres of Remak, include those that are devoid of the myelin sheath 

 throughout their course. Such fibres are chiefly the axones of sympathetic 

 neurones. They are often 2 /j. or less in diameter and consist of only the 

 axis-cylinder and the neurilemma, the latter being thin and delicate. The 

 pale fibres, like others, end in terminal arborizations (telodendria) composed 

 of naked axis-cylinders. 



N euroglia. The neurones (nerve-cells and nerve-fibres) within the brain 

 and spinal cord are everywhere held together by a special supporting tissue 

 known as neuroglia. The latter is derived primarily from the invaginated 

 ectoderm which forms the wall of the neural tube, certain elements, the spongio- 

 blasts, being concerned in the production of the neuroglia, while others, the 

 neiiroblasts , give rise to the neurones. For a time the supporting tissue is 

 represented by greatly elongated radially disposed fibre-cells that often ex- 

 tend the entire thickness of the wall of the neural tube. Later, the neurog- 

 liar elements become differentiated into: (a) those bordering the lumen of 

 the canal, where they are partially retained as the ependymal cells; and () 

 those which early migrate to more peripheral positions and give rise to stel- 

 late gliogenetic cells that are converted into spider-like elements (Fig. 95). 

 In chrome-silver preparations these appear as irregular triangular or quadri- 



FIG. 94. Nonmedullated 

 nerve fibres in longitudinal 

 section of splenic nerve. 

 X3io. 



