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TifA' NERVOUS SYSTEM. 267 ;

Fig. 123.

A, tubular nerve-fibres, showing the sinuous outline and double
contours ; li, diagram to show the parts of a tubular fibre—viz.
1, 1, membranous tube; 2, 2, white substance or medullary Nerve-fibres, fixed and stained
sheath; 3, axis or primitive band; C, figure (imaginary) in- by perosmic acid, from the
tended to represent the appearances occasionally seen in the posterior wall of dorsal
1, 1, membrane of the tube seen at parts where lymph-sac
tubular fibres: of frog: 1, ],
the white substance has separated from it ; 2, a part where the medullary layer; 2, axis-cyl-
white substance is interrupted; 3, axis projecting beyond the inder; 3, 3, constrictions "of
broken end of the tube 4, part of the contents of "the tube Rauvier; 4, 4, incisions of
;
escaped. t^chmidt.
The Neurilemma or Sheath of Schicann i.s the outer covering of a
nerve-fibre, and forms a continuous envelope ; a narrow lymph-space
extends between it and the medullary sheath. It is the analogue of
the sarcolemma in a muscular fibre, and appears as a fine hyaline,
homogeneous, elastic membrane, with flattened or oval-shaped nucleated
corpuscles, known as nerve-corpuscles, situated between it and the
medullary sheath. The nucleus is generally seen in a depression of the
medullary membrane surrounded by a zone of granular protoplasm
this is especially the case in young subjects. The optic and auditory
nerves have no neurilemma.
The Nodes and Internodes of Banvier (Fig, 124) are cau.sed by the
annular constriction or breaks in the continuity of the medullary sheath
or white substance of Schwann. The axi.s-cylinder, the neurilenmia, and
the lymph-spaces are not interrupted at these points, though the neuri-
lemma curves sharply inward and comes in close apposition to the axis-
cylinder. The point at which the constriction takes place is named the
node of Ranvier, and the portions between, the internodes of Banvier or
interannular segments. Each interiiode or segment has usually one or
more nerve-corpuscles situated between the medullary sheath and the
neurilemma.
A fresh nerve treated with a solution of nitrate of silver or osmic
acid and exposed to the light demonstrates distinctly the nodes of
Eanvier (Fig. 125). After long exposure the silver salt penetrates