46 O B S E R V A T I O N S ON THE
and extenfors of the fore-arm ; it is evidently better for us, that the one
half of each nerve go'es to the flexors, and the other half of each to the
extenfors, than that the whole of the firft nerve ihould have gone to the
flexors, and the whole of the fécond to the extenfors. For if, by accident
or difeafe, one of thefe nerves fhould be cut acrofs, or lofe its powers, we
fhould, on the firft fuppofition, preferve one half of the power both of
flexion and extenfion, which would furely be preferable to our poiTeffing
fully tlie flexion, without any power of extenfion.
If a fl:iU greater number of nerves is employed to fupply the flexors and
extenfors, the lofs of power, arifing from an accident happening to one of
the trunks of the nerves, will be felt in a much fmaller degree ; thus, in
the arm, where five trunks are found, a fifth part only of the power would
be loft.
T . III.
1. NOTWITHSTANDING we have obferved that the fibres of the nerves, in
their courfe, are much more intimately intermixed than has been fuppofed,
ftill, as their branches do not anaftomofe, there is little or no reafon for
believing that the energy of one fibre can diretìly afifeft that of a neighbouring
or contiguous fibre, or that the fympathy of nerves can depend
directly on their conneftion in their progrefs. We are, therefore, led to
refer it, in the firft place, to their connections in the brain, where we have
found there are many more medullary fibres than are fufficient to form a
bulk equal to all the nerves, and where, befides, many of the fibres feenl
evidently intended to conneft oppofite fides of the brain.
2. But although we cannot, from the intermixture of the nervous fibres
in their courfe, account for their fympatliy, but are obliged to refer the
caufe to their connexions in the brain, in which the feeling principle is
feated ; yet, from finding that each fmall branch of a nerve is derived from
various fources, we perceive, that an injury done to it may afieél and irritate
N E R V O U S S Y S T E M . 47
tate the brain in various and diftant places, and, thefe reading, we underftand
better how a fuffcrance by fympathy may become extenfive or univerfal,
than we do, when we conceive that each nerve is derived from a
. fingle fmall portion only of the brain.
3. As different nerves, intermixed, are fupplied by branches of the fame
artery, and that the arteries furnilh pia mater and cortical matter to the
nerves in their progrefs, and evidently influence their energy, it may be a
quefluon meriting attention, whether fympathy of nerves may, in fome
meafure, depend on the irritation and reaition of their accompanying bloodveflfels
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C H A P .