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THE NERVOUS SYSTEM. 269

other trunks, thus forming a single bundle arising from two or more
sources and possessing two or more functions ; or they may break up
and unite in various ways, forming plexuses, as the brachial or cer-
vical.
The medullary fibres while in the nerve-cords or in the nerve-centres
do not branch or unite with each other ; when near their termination
it is claimed they occasionally do so, in which case
Fig. 120.
the branches are always at one of the nodes of
Ranvier (Fig. 127). The new axis-cylinder thus
formed has its own medullary sheath and neuri-
lemma, being a continuation of the covering of the
nerve-fibre from which it originates.
As the nerves ai)proacIi their termination they
^
divide and subdivide into bundles, until they be-
come very minute, and consist of a single bundle
of a few fibres encased in a perineurium made of

Fig. 128.

Fig. 127.

Portion of the Xetwork of
Fibres of Keniak, from
the pneuniogastric of
the dog: 7(, nucleus: /), Division of a Nerve-
protoplasm surrounding fibre, from pulmo-
it; h, striation caused nary membrane of I)ivision of a Nervous l^ranch (a) into its
by fibrils. frog's lung. ultimate fibres, h, c, d, e.
a delicate film of connective tii^sue (Fig. 128). Finally, the nerve
becomes a single medullated fibre, which soon loses its coat, exposing
the axis-cylinder ; this ultimately breaks up into ])rimitive nerve-fibrillse.
These become beaded, branching and uniting with each other, and form-
ing a very fine network, the density of which is dependent upon the
number of nerve-fibres distributed to the parts ; thus, in some portions