

STRUCTURE OF THE NERVOUS SYSTEM 



727 





at these points: these are known as the nodes of Ranvier (Figs. 631 and 633). The 

 portion of nerve fiber between two nodes is called an internodal segment. The 

 neurolemma or primitive sheath is not interrupted at the nodes, but passes over 

 them as a continuous membrane. If the fiber be treated with silver nitrate the 

 reagent penetrates the neurolemma at the nodes, and on exposure to light reduction 

 takes place, giving rise to the appearance of black crosses, Ranvier's crosses, on the 

 axis-cylinder. There may also be seen transverse lines beyond the nodes termed 

 Frommann's lines (Fig. 634) ; the significance of these is not understood. In addi- 

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

 subdividing it into irregular portions, which are termed medullary segments, or 

 segments of Lantermann (Fig. 631); there is reason to believe that these clefts are 

 artificially produced in the preparation of the specimens. Medullated nerve 

 fibers, when examined in the fresh condition, frequently present a beaded or vari- 

 cose 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. 



Node of ^ 

 Sanvitr 



Frommann's 

 linet 



FIG. C34. Medullated nerve fibers stained with silver nitrate 



FIG. 635. A small nervous branch 

 from the sympathetic of a mammal. 

 a. Two medullated nerve fibers among 

 a number of gray nerve fibers, b. 





The neurolemma or primitive sheath presents the appearance of a delicate, 

 structureless membrane. Here and there beneath it, and situated in depressions 

 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 center of each 

 internode. The primitive sheath is not present in all medullated nerve fibers, 

 being absent in those fibers which are found in the brain and medulla spinalis. 



Wallerian Degeneration. When nerve fibers are cut across, the central ends of the fibers 

 degenerate as far as the first node of Ranvier; but the peripheral ends degenerate simultaneously 

 throughout their whole length. The axons break up into fragments and become surrounded by 

 drops of fatty substance which are formed from the breaking down of the medullary sheath. 

 The nuclei of the primitive sheath proliferate, and finally absorption of the axons and fatty 

 substance occurs. If the cut ends of the nerve be sutured together regeneration of the nerve 

 fibers takes place by the downgrowth of axons from the central end of the nerve. At one time 

 it was believed that the regeneration was peripheral in origin, but this has been disproved, the 

 proliferated nuclei in the peripheral portions taking part merely in the formation of the so-called 

 scaffolding along which the new axons pass. 





