SPINAT. MARROW. . £47 
preserve their form. Besides these, there is a deiicate cellu- 
lar membrane, containing bloodvessels, which penetrates the 
interior, and forms sheaths round the component filaments. 
This membrane is termed Neurilema. It may be exhibited 
by dissolving the substance of the nerve in caustic potash ; 
or the existence of filaments may be shewn by removing the 
neurilema by acids. In this last case, they may be observed 
anastamosing in various ways. 
Some of the nerves appear to have a simple origin ; but 
in general, several filaments, from different parts of the 
brain or spinal marrow, unite to form the trunk of a nerve. 
This trunk again subdivides in various ways; but the ra- 
mifications do not always exhibit a proportional decrease of 
size. It frequently happens, that the branches of the same 
or of different nerves, unite and separate repeatedly within 
a small space, forming a close kind of net-work, to which the 
name Plexus has been applied. Sometimes filaments pass 
from one nerve to another; and, at the junction, there is 
usually an enlargement of medullary matter termed a Gan- 
glion. Numerous filaments, from different nerves, often 
unite to form a ganglion, from which proceed trunks fre- 
quently of greater magnitude than the filaments which en- 
tered. Thus nerves, very different in their origin, form 
communications with one another; so that the whole ner- 
vous system, may be considered as a kind of net-work, be- 
tween the different parts of which, an intimate connection 
subsists. In consequence of this arrangement, it is often mat- 
ter of very great difficulty to ascertain the origin of those 
filaments which unite to constitute the trunk of a nerve. In 
some instances, they appear to arise from the surface of the 
brain or spinal marrow; in other cases, from the more cen- 
tral parts. 
The manner in which the nerves terminate in the diffe- 
rent organs to which they are distributed, is still consi- 
K 2 
~ 
