Injuries of nerves and their consequences - Reflex Sympathetic ...

ANATOMY OF NERVES. 27
the periphery, from which in many cases filaments emerge;
while in other instances these loops are merely recurrent
communicating branches. Hyrtl has given numerous
instances of these. Some of the more familiar are the
loops of the hypoglossal, the return upwards of the recurrent
laryngeal, and the palmar or plantar arcades. The
chiasm of nerves on opposite sides of the body
is more
rare. That of the optic nerves in man seems to be in
him the sole instance of a true physiological chiasm,
since Vulpian* thinks he has proved that in the case of
the intercommunication of the right and left hypoglossals
pointed out by Hyrtl, there is no transmission of power
from side to side; and, indeed, if I correctly understand
the passage, he would seem to have some doubt as to
whether there is really an interchange of fibres.
In man the cerebro-spinal nerves, at least, seem to be
physiologically limited to their own side of the body, a
proposition which becomes of some importance in the
recognition and limitation of paralytic affections. I have
once or twice felt doubtful as to the strict truth of this
law as applied to certain traumatic palsies of the face,
where in rare cases the motor fibres of the upper lip have
appeared to me to cross over, so that irritation of the
nerve on the sound side seemed to cause slight muscular
contractions on the diseased side.
In the lower animals, the existence of any true physiological
chiasm, other than that of the optic nerves, was first
demonstrated by the author, and his friend Dr. Morehouse,
in the case of the chelonians, all of which have a
chiasm between the two superior laryngeal nerves, sotnat
irritation of the left nerve acts on both the left and right
lips of the larynx, and rice versa. Guided by this dis-
found similar chiasms in
covery, my friend Prof. Wyman
* Yulpian, Le9ons sur la Phys. Comp., etc., p. 160.