DETERMINATION OF NEURAL CONNECTIONS
199
ter. This conclusion obtained further confirmation from the experiment
in which the distal end of the cut ophthalmic nerve was cross-united
with the central end of the mandibular nerve in the newt (Sperry and
Miner, 1949). After nerve regeneration, ophthalmic-type reflexes could
be evoked from the mandibular region—i.e., when the skin of the lower
jaw was mechanically stimulated with a needle, the animal pressed the
skin against the needle by bending the head downward, instead of
lifting up its head as a normal animal did to avoid pin pricks. This
result led the authors to the conclusion that the trigeminal sensory cells,
like the retinal nerve cells, were specified according to their terminations
in the skin and established selective connections with the similarly
specified, secondary, sensory neurons.
To investigate the embryonic development of the specificity of cranial
sensory ganglia, reflexes evoked from head regions were investigated in
newt embryos following the replacement of the primordium of the
trigeminal ganglion by that of the vagus ganglion, or vice versa (Székely,
1959a). The head skin posterior to the ear capsule is supplied by vagal
sensory nerves; their characteristic reflex is depression of the gills, which
can be evoked during larval life. The rest of the head skin is innervated
by the trigeminal nerve, and its characteristic reflex is the eye-withdrawal
(corneal) reflex which can be evoked only after metamorphosis, when the
gills disappear and the eyelids develop. The results are summarized in
Fig. 3. In about one-half of the animals with two vagus ganglia, the gill
reflex could be evoked from the cornea as well, which indicates that the
grafted vagus ganglion had somehow managed to establish connections
with the vagus motor nucleus. After metamorphosis, normal corneal
reflexes could be elicited, and in a few cases the reflexes appeared to the
stimulation of the normal vagus sensory area as well. The result indicates that both the grafted and the normal vagus ganglia made connections with the abducens nucleus. In the other group also about one-half
of the animals with two trigeminal ganglia yielded gill reflexes from
the posterior part of the head and from the cornea as well. It can be
inferred that both trigeminal ganglia established connections with the
vagus motor nucleus. Beside the normal corneal reflex, the only interesting phenomenon recorded after metamorphosis was a head jerk when
strong stimuli were applied to the posterior part of the head. In a third
group normal reflexes developed after a "mock operation" by changing
the trigeminal ganglion for another.
To interpret the results one may atribute a small degree of specificity
to the ganglion cells, which, however, does not prevent them from making
contact with an inappropriate motor nucleus (trigeminal-vagus, vagusabducens) as well. Nevertheless, an auxiliary theory is required to ex-
199
ter. This conclusion obtained further confirmation from the experiment
in which the distal end of the cut ophthalmic nerve was cross-united
with the central end of the mandibular nerve in the newt (Sperry and
Miner, 1949). After nerve regeneration, ophthalmic-type reflexes could
be evoked from the mandibular region—i.e., when the skin of the lower
jaw was mechanically stimulated with a needle, the animal pressed the
skin against the needle by bending the head downward, instead of
lifting up its head as a normal animal did to avoid pin pricks. This
result led the authors to the conclusion that the trigeminal sensory cells,
like the retinal nerve cells, were specified according to their terminations
in the skin and established selective connections with the similarly
specified, secondary, sensory neurons.
To investigate the embryonic development of the specificity of cranial
sensory ganglia, reflexes evoked from head regions were investigated in
newt embryos following the replacement of the primordium of the
trigeminal ganglion by that of the vagus ganglion, or vice versa (Székely,
1959a). The head skin posterior to the ear capsule is supplied by vagal
sensory nerves; their characteristic reflex is depression of the gills, which
can be evoked during larval life. The rest of the head skin is innervated
by the trigeminal nerve, and its characteristic reflex is the eye-withdrawal
(corneal) reflex which can be evoked only after metamorphosis, when the
gills disappear and the eyelids develop. The results are summarized in
Fig. 3. In about one-half of the animals with two vagus ganglia, the gill
reflex could be evoked from the cornea as well, which indicates that the
grafted vagus ganglion had somehow managed to establish connections
with the vagus motor nucleus. After metamorphosis, normal corneal
reflexes could be elicited, and in a few cases the reflexes appeared to the
stimulation of the normal vagus sensory area as well. The result indicates that both the grafted and the normal vagus ganglia made connections with the abducens nucleus. In the other group also about one-half
of the animals with two trigeminal ganglia yielded gill reflexes from
the posterior part of the head and from the cornea as well. It can be
inferred that both trigeminal ganglia established connections with the
vagus motor nucleus. Beside the normal corneal reflex, the only interesting phenomenon recorded after metamorphosis was a head jerk when
strong stimuli were applied to the posterior part of the head. In a third
group normal reflexes developed after a "mock operation" by changing
the trigeminal ganglion for another.
To interpret the results one may atribute a small degree of specificity
to the ganglion cells, which, however, does not prevent them from making
contact with an inappropriate motor nucleus (trigeminal-vagus, vagusabducens) as well. Nevertheless, an auxiliary theory is required to ex-