;
1 9
O B S E R V A T I O N S ON THE
C H A P T E R XV.
Of the Plexufes of Nerves.
EVERY nerve of the fpinal marrow is joined near its exit, by an arch,
with the nerve above and below it: But the connedion of the feveral
nerves of the arm, or of the leg, may be confidered as chief examples of
plexus.
Euftachius, and feveral fucceeding authors, have given pretty exaft delineations
of the outfide appearance of thefe plexufes ; but no author has
either carefully traced the nervous fibres in their courfe through them, or
properly explained the ufes they are intended to ferve.
Many years ago, I was at tlie pains of tracing the fibres of the nerves of
the arm from their origin in the fpinal marrow, to their termination in the
parts of the arm, by flitting open the flieaths of all the trunks where they
form the brachial plexus. I found, as, indeed, I expeSed, that in the
plexufes, the fibres of the diflpcrent trunks were intermixed, and that every
nerve under the plexus confifted of fibres of all the nerves, which were tied
together above its origin from the plexus
The joining of the optic nerves not only refembles, in its outfide appearance,
the joining of two of the cervical nerves in the brachial plexus,
but it is impoffible to feparate them, without tearing the medullary cords ;
fo that there appears to me no doubt, that the medullary fibres, from the
two fides of the brain, are there intermixed. But as this nerve is more
purely medullai-y than nerves in general are in their courfe, and fofter, becaufe
• See Tab. XV. Tnb. XVI. Tab. XVII.
N E R V O U S SYSTElVi. 4i
caufe its component fibres are lefs feparated by the intervention of the pia
mater, it is much more difficult to trace its fibres from end to end, fo as to
demonftrate the fad clearly: But, unlefs I am much deceived, I have feen,
in man, an intermixture and partial decuITation of the cords which compofe
the optic nerves
C H A P T E R XVI.
Of the Conneftion of Nerves which run in oppofite Directions,
fo as to be joined by their fmall Branches.
" ' H E moil remarkable conneaion of this kind is, in the human face,
where the portio dura of the auditory nerve is joined to the fécond
and third branches of the fifth pair of nerves.
Many of thefe nerves joined are fo fmall, that it is very difficult to trace
the fibrils of which they confift : But, in the larger branches, both in man
and in the ox, I have certainly found, that the fmall branches which come
off beyond the union of two larger nerves, confift of fibres from both f .