204
GEORGE SZÉKELY
closure, effected through the abducens nerve. Since the trigeminal nerve
is the afferent limb of the reflex, this could be evoked from the whole
sensory area of the ophthalmic division of the trigeminal, the area extending from the snout to the ear vesicle. The threshold for evoking the
reflex is lowest by far on the cornea, a little higher on the lids, and rises
steeply toward the border of the reflexogenic zone. After grafting an extra
eye into the snout in salamanders, or into the place of the ear vesicle
(from which a corneal reflex cannot usually be evoked), Weiss (1942,
1950) found withdrawal of the host's own ipsilateral eye upon touching
the extra cornea. The threshold was just as low as on the normal cornea.
In the absence of extraocular muscles, the grafted eye remained motionless. Weiss interpreted the result as showing that the nerves invading the
grafted eye became modulated by the cornea, and the central processes
changed their reflex relations according to this specific corneal
modulation.
Since the eye was grafted into the fringe of the reflexogenic zone, the
question arises whether a considerable decrease of the threshold, caused
by the surgical injury, gave rise to the corneal reflex rather than a
specific modulation. Weiss himself also raised this possibility and excluded it on the basis of the observation that injuries in the ear region
increased the sensitivity only temporarily, and that after healing no
responses could be evoked from the injured region. Since the tests were
made several months after operation, a mere injury, therefore, could not
account for the high sensitivity of the extra cornea in evoking the reflex.
One possibility, however, remains, namely, that the grafted eye may have
collected a large number of regenerating fibers during the healing period.
These, by secondary arborization, could produce a higher concentration
of terminals in the cornea than in the surrounding skin, and so the cornea
might act as a permanent wound surface in lowering the threshold of
the respective area.
Working on the development of the corneal reflex, Kollros (1943) demonstrated that an eye withdrawal could be elicited from eyes grafted
behind the ear region of Ambystoma,
an area which is definitely out of
the normal reflexogenic region supplied by vagus sensory fibers. Similar
results were found also in frogs (Kollros, 1943) and in
Pleurodeles
(Székely, 1959b), and these seemed to support Weiss's idea about a
specific corneal modulation of the involved sensory neurons. However,
while studying reflexes elicited from extra limbs grafted into the vagus
sensory area in Pleurodeles and Tnturus,
Székely (1959b) found that
a corneal reflex could be evoked from a regeneration blastema produced
on the distal end of the grafted limb by cutting off the foot in animals
just before metamorphosis. As the regeneration of the foot proceeded,
GEORGE SZÉKELY
closure, effected through the abducens nerve. Since the trigeminal nerve
is the afferent limb of the reflex, this could be evoked from the whole
sensory area of the ophthalmic division of the trigeminal, the area extending from the snout to the ear vesicle. The threshold for evoking the
reflex is lowest by far on the cornea, a little higher on the lids, and rises
steeply toward the border of the reflexogenic zone. After grafting an extra
eye into the snout in salamanders, or into the place of the ear vesicle
(from which a corneal reflex cannot usually be evoked), Weiss (1942,
1950) found withdrawal of the host's own ipsilateral eye upon touching
the extra cornea. The threshold was just as low as on the normal cornea.
In the absence of extraocular muscles, the grafted eye remained motionless. Weiss interpreted the result as showing that the nerves invading the
grafted eye became modulated by the cornea, and the central processes
changed their reflex relations according to this specific corneal
modulation.
Since the eye was grafted into the fringe of the reflexogenic zone, the
question arises whether a considerable decrease of the threshold, caused
by the surgical injury, gave rise to the corneal reflex rather than a
specific modulation. Weiss himself also raised this possibility and excluded it on the basis of the observation that injuries in the ear region
increased the sensitivity only temporarily, and that after healing no
responses could be evoked from the injured region. Since the tests were
made several months after operation, a mere injury, therefore, could not
account for the high sensitivity of the extra cornea in evoking the reflex.
One possibility, however, remains, namely, that the grafted eye may have
collected a large number of regenerating fibers during the healing period.
These, by secondary arborization, could produce a higher concentration
of terminals in the cornea than in the surrounding skin, and so the cornea
might act as a permanent wound surface in lowering the threshold of
the respective area.
Working on the development of the corneal reflex, Kollros (1943) demonstrated that an eye withdrawal could be elicited from eyes grafted
behind the ear region of Ambystoma,
an area which is definitely out of
the normal reflexogenic region supplied by vagus sensory fibers. Similar
results were found also in frogs (Kollros, 1943) and in
Pleurodeles
(Székely, 1959b), and these seemed to support Weiss's idea about a
specific corneal modulation of the involved sensory neurons. However,
while studying reflexes elicited from extra limbs grafted into the vagus
sensory area in Pleurodeles and Tnturus,
Székely (1959b) found that
a corneal reflex could be evoked from a regeneration blastema produced
on the distal end of the grafted limb by cutting off the foot in animals
just before metamorphosis. As the regeneration of the foot proceeded,