DETERMINATION OF NEURAL CONNECTIONS
207
to those evoked from corresponding points of the normal limb. This result
was interpreted as showing that cutaneous fibers, normally destined to
form synapses appropriate for the belly, flank, and dorsal skin of the
trunk, instead formed connections appropriate for the digits, heel, and
knee of the limb.
This problem was investigated using chickens (Székely and Szentâgothai, 1962). On the third day of incubation, limb buds, both legs and
wings, were excised from chick embryos and implanted into the midthoracic region of other embryos of the same age. Such limbs remained
motionless, but their stimulation gave rise to various responses of the
host's own limbs after hatching. Among the responses, two types could
generally be distinguished. A simple form of response, upon mechanical
stimulation of the graft, was a flexion reflex of the normal leg on the side
from which the graft received nerves. Irrespective of whether a wing or
a leg had been transplanted, the chick always responded with the limb
nearest to the graft. When a wing response occurred, it consisted of
elevation of the wing. Sometimes the grafts were insensitive to pain (no
pain reactions such as loud chirping and escaping movements could be
evoked), although limb-specific reflexes could be elicited from the distal
end of the transplanted leg covered with scaly skin. In one case, a transplanted wing was sensitive to pain but yielded no reflexes. The reflexes
could clearly be shown about 3 days after hatching. Although these reflexes were present in all the animals with successful grafts, only some
of them (four out of nine) showed complex behavioral responses. The
simplest and most common form of this sort of response was when, after
stronger pressure on the transplanted limb, the chick started to walk
lamely, frequently elevating its own leg on the side of the graft, as if
feeling severe pain in the elevated leg. More impressive were the characteristic cleaning and disentangling movements made with the beak, as
if to remove some irritation from the normal ipsilateral leg. These responses could be sustained by applying a steel clamp to the graft, which
caused a prolonged painful stimulation. However, having caught sight of
the clamp, the chick could localize it correctly. These complex behavioral
responses gradually emerged between the fifth and eighth day after
hatching and could be provoked only from a few specific points, whereas
the chick yielded simple reflexes from the entire surface of the graft.
Dissection revealed that the nerves from two or three thoracic segments
richly innervated the grafts, and, in cases exhibiting complex behavioral
patterns, the neighboring nerves formed a plexus before entering the
graft.
Difficulties arise when one tries to interpret the results on the basis of
functional specificity. All these types of responses could be elicited from
207
to those evoked from corresponding points of the normal limb. This result
was interpreted as showing that cutaneous fibers, normally destined to
form synapses appropriate for the belly, flank, and dorsal skin of the
trunk, instead formed connections appropriate for the digits, heel, and
knee of the limb.
This problem was investigated using chickens (Székely and Szentâgothai, 1962). On the third day of incubation, limb buds, both legs and
wings, were excised from chick embryos and implanted into the midthoracic region of other embryos of the same age. Such limbs remained
motionless, but their stimulation gave rise to various responses of the
host's own limbs after hatching. Among the responses, two types could
generally be distinguished. A simple form of response, upon mechanical
stimulation of the graft, was a flexion reflex of the normal leg on the side
from which the graft received nerves. Irrespective of whether a wing or
a leg had been transplanted, the chick always responded with the limb
nearest to the graft. When a wing response occurred, it consisted of
elevation of the wing. Sometimes the grafts were insensitive to pain (no
pain reactions such as loud chirping and escaping movements could be
evoked), although limb-specific reflexes could be elicited from the distal
end of the transplanted leg covered with scaly skin. In one case, a transplanted wing was sensitive to pain but yielded no reflexes. The reflexes
could clearly be shown about 3 days after hatching. Although these reflexes were present in all the animals with successful grafts, only some
of them (four out of nine) showed complex behavioral responses. The
simplest and most common form of this sort of response was when, after
stronger pressure on the transplanted limb, the chick started to walk
lamely, frequently elevating its own leg on the side of the graft, as if
feeling severe pain in the elevated leg. More impressive were the characteristic cleaning and disentangling movements made with the beak, as
if to remove some irritation from the normal ipsilateral leg. These responses could be sustained by applying a steel clamp to the graft, which
caused a prolonged painful stimulation. However, having caught sight of
the clamp, the chick could localize it correctly. These complex behavioral
responses gradually emerged between the fifth and eighth day after
hatching and could be provoked only from a few specific points, whereas
the chick yielded simple reflexes from the entire surface of the graft.
Dissection revealed that the nerves from two or three thoracic segments
richly innervated the grafts, and, in cases exhibiting complex behavioral
patterns, the neighboring nerves formed a plexus before entering the
graft.
Difficulties arise when one tries to interpret the results on the basis of
functional specificity. All these types of responses could be elicited from