THE NERVOUS TISSUE. 



1117 



considerable extent, in some forming a layer of extreme thinness, so as to be 

 scarcely distinguishable ; in others forming about one-half the nerve-tube. The 

 size of the nerve-fibres, which varies from ^wo" ^ 12^0 ^ an i ncn > depends 

 mainly upon the amount of the white substance, though the axis-cylinder also 

 varies in size within certain limits. The white matter of Schwann does not always 

 form a continuous sheath to the axis-cylinder, but undergoes interruptions in its 

 continuity at regular intervals, giving to the fibre the appearance of constriction 

 at these points. These were first described by Ranvier, and are known as the 

 nodes of Ranvier (Fig. 654). The portion of nerve-fibre between two nodes is 

 called an internodal segment. The neurilemma or primitive sheath is not inter- 

 rupted at the nodes, but passes over them as a continuous membrane. In addition 

 to these interruptions oblique clefts may be seen in the medullary sheath, sub- 

 dividing it into irregular portions, which are termed medullary segments, or seg- 

 ments of Lantermann (Fig. 658). There is reason to believe that these clefts are 

 artificially produced in the preparation of the specimens. Medullated nerve-fibres, 

 when examined, frequently present a beaded or varicose appearance : this is due 

 to manipulation and pressure causing the oily matter to collect into drops, and in 

 consequence of the extreme delicacy of the primitive sheath, even slight pressure 

 will cause the transudation of the fatty matter, which collects as drops of oil outside 

 the membrane. This is, of course, promoted by the action of certain reagents. 



The neurilemma or primitive sheath (sometimes called the tubular membrane or 

 sheath of Schwann) presents the appearance of a delicate, structureless membrane. 

 Here and there beneath it, and situated in depres- 

 sions in the white matter of Schwann, are nuclei 

 surrounded by a small amount of protoplasm. The 

 nuclei are oval and somewhat flattened, and bear a 

 definite relation to the nodes of Ranvier ; one 

 nucleus generally lying in the centre of each inter- 

 node. The primitive sheath is not present in all 

 medullated nerve-fibres, being absent in those fibres 

 which are found in the brain and spinal cord. 



Non-medullated Fibres. Most of the nerves of 

 the sympathetic system, and some of the cerebro- 

 spinal, consist of another variety of nervous fibres, 

 which are called the gray or gelatinous nerve-fibres 

 fibres of Remak (Fig. 655). These consist of a 

 central core or axis-cylinder enclosed in a nucleated 

 sheath, which tends to split into fibrillse, and is 

 probably of the nature of neurokeratin. In exter- 

 nal appearance the gelatinous nerves are semi- 

 transparent and gray or yellowish-gray. The 

 individual fibres vary in size, generally averaging about half the size of the 

 medullated fibres. 



Development of Nerve-cells and Fibres. The nerve-cells are developed from 

 certain of the cells which line the neural canal or form the neural crest of the 

 embryo (see section on Development). Some of these cells assume a rounded form 

 and are termed neuroblasts, and from each neuroblast there grows out a process, 

 the axis-cylinder process or axon, and subsequently the branching processes or 

 dendrons. The axis-cylinders, at first naked, acquire their medullary sheath, 

 possibly by some metamorphosis of their outer layer. The neurilemma is thought 

 to be derived from mesoblastic cells which become flattened and wrapped round 

 'he fibre, the cement-substance at their apposed ends forming the material which 

 stains with silver nitrate at the nodes of Ranvier. Nerve-cells in the sympathetic 

 and peripheral ganglia take their origin from small collections of neuroblasts, 

 .vhich are split off from the rudimentary spinal ganglia. Cells which are, origi- 

 nally, similar to neuroblasts seem to give rise to neuroglia-cells, numerous processes 

 sprouting from the cell to form the neuroglial fibres. 



FIG. 655. A small nervous branch 

 from the sympathetic of a mammal, 

 a. Two medullated nerve-fibres among 

 a number of gray nerve-fibres, b. 



