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ZOOLOGY: S. R. DETWILER 
may be almost entirely limited to co-ordinated movements of the limbs. 
Such responses may be carried out perfectly by the transplanted limbs 
when their peripheral innervation is derived from the normal limb 
level of the cord, but the ability of the transplanted limbs to exhibit 
movements, co-ordinated with the opposite intact limb, decreases mark- 
edly when their peripheral innervation is derived from segments well 
beyond the normal limb level (series AS6). 
As has been shown by Herrick^ we have developed here a central 
nervous architecture by means of vv^hich peripheral sensory stimuli pass 
through more or less localized ascending sensory tracts from the cord 
to the medulla (tractus spino-bulbaris) , to the midbrain (tractus spino- 
tectalis) and to the thalmus (tractus spino-thalamicus) . These stimuli 
may become finally discharged into the somatic motor centers of the 
spinal cord by means of descending tracts such as the tractus thalamo- 
bulbaris, tractus tecto-bulbaris, the fasciculus longitudinalis medialis 
and the tractus bulbo-spinalis. According to Herrick the cell bodies 
of the tractus bulbo-spinalis lie in the general motor tegementum of the 
medulla and their axones are directed ventrally into the ventral funiculi 
of the same and the opposite side. It is highly probable that a certain 
number of these fibers normally develop only as far as the third, fourth 
and fifth segments of the cord for specific discharge into the normal 
appendicular somatic motor centers. The fact that transplanted limbs, 
receiving peripheral innervation from these levels, exhibit movements 
which are co-ordinated with the opposite intact limb, strongly suggest 
such a condition. The behavior of limbs innervated mainly from the 
sixth, seventh and eighth segments of the cord (series ASS) suggests 
that these descending neurones, which normally end in the limb level, 
may be induced to continue their growth an additional segment or two 
to meet the functional demands imposed upon them by the trans- 
planted limb. Their incapacity for further functional regulation is 
suggested by the loss of co-ordinated function and the greatly impaired 
movements that are exhibited by limbs of the series AS6, probably 
none of which receive peripheral innervation from segments of the cord 
anterior to the eighth. 
The increase in the number of cases with total loss of function as the 
limbs are implanted more and more posteriorly (table 1 A) would also 
suggest that there occurs a corresponding increased deficiency in the 
connections of the purely intraspinal correlation neurones. 
Additional limbs transplanted respectively three, four and five seg- 
ments posterior to the normal intact limb of the host (table 1 B), never 
