160 Ss. R. DETWILER 
brain (tractus spinotectalis), and to the thalamus (tractus spino- 
thalamicus). These stimuli may become finally discharged into 
the somatic motor centers of the spinal cord by means of de- 
scending tracts such as the tractus thalamobulbaris, tractus tecto- 
bulbaris,the fasciculus longitudinalis medialis, and the tractus 
bulbospinalis. According to Herrick (op. cit.), the cell bodies 
of the tractus bulbospinalis lie in the general motor tegmentum 
of the medulla and their axones are directed ventrally into the 
ventral funiculi of the same and opposite sides. It is highly 
probable that a certain number of these fibers normally develop 
only as far as the third, fourth, and fifth sgements of the cord 
for specific discharge into the normal appendicular somatic 
motor centers. The fact that transplanted limbs, which receive 
peripheral innervation from these levels, do exhibit movements 
coordinated with the opposite intact limb strongly suggests 
such a condition. The behavior of limbs innervated mainly 
from the sixth, seventh, and eighth segments of the cord (series 
AS5) suggests that these descending neurones, which normally 
end in the limb level, may be induced to continue their growth 
an additionals egment or two in order to meet the functional 
demands imposed upon them by the transplanted limb. Their 
incapacity for further functional regulation is suggested by the 
loss of coérdinated function and the greatly impaired movements 
such as are exhibited by limbs of the series AS6 (table 1 a)—prob- 
ably none of which receive peripheral innervation from segments 
of the cord anterior to the eighth (table 2). 
The increase in the number of cases with total loss of function 
as the limbs are implanted more and more posteriorly (table 1a 
and fig. 7) would also suggest that, in addition to an inadequate 
supply of descending neurones, there probably occurs a corre- 
sponding increase deficiency in the connections of purely intra- 
spinal correlation neurones. 
Additional homoplastic limbs transplanted respectively three, 
four, and five segments posterior to the normal intact limb of 
the host (table 1b), never attain the completeness of function 
attained by autoplastic limbs transplanted to the same relative 
positions. Such limbs may be well supplied with peripheral 
