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Cruce 
of the spinal gray, bilaterally, and acts upon 
interneurons and motoneurons controlling 
axial and proximal limb musculature. 
Fibers from precentral motor cortex may 
overlap the spinal terminations of the brain- 
stem pathway (Kuypers, 1973). In some pri- 
mates corticospinal fibers terminate in both 
the lateral and medial regions of the gray, 
thus allowing the forebrain to have a strong 
direct effect upon the activity of spinal inter- 
neurons and motoneurons controlling both 
limb and axial musculature. In other mam- 
mals, corticospinal fibers terminate only in 
the lateral spinal region, as does the lateral 
brainstem pathway. Thus in these animals 
only limb musculature is under direct control 
of the forebrain. 
Reptiles lack the corticospinal pathway 
and probably have no cortical efferents ter- 
minating in the brainstem (Ariens Kappers, 
et al., 1936 ; Goldby, 1937 ; Goldby and 
Gamble, 1957 ; Kruger and Berkowitz, 1960). 
On the other hand, they possess all of the 
mammalian brainstem structures which pro- 
ject to the spinal cord (Ariens Kappers, et 
al., 1936; Robinson, 1969; Cruce and Nieu- 
wenhuys, 1974; Cruce, et al., 1976; Donke- 
laar, 1976a,6). Thus reptiles offer the possi- 
bility for studying the function and structure 
of supraspinal motor control in a system un- 
complicated by cortical descending pathways. 
The range of variation in both morphology 
and methods of locomotion to be found 
amongst the reptiles provide a number of 
interesting experimental models which we 
have only just begun to explore. While turtles 
have no trunk musculature and snakes (and 
some lizards) have no limbs, most lizards 
have both limb and trunk musculature which 
are employed in locomotion. Thus reptiles 
provide a number of experimental models 
for separating the functions of the medial 
and lateral brainstem pathways. Recently I 
have focused attention upon the Tegu lizard, 
a reptile which is a good example of a gener- 
alized terrestrial tetrapod and therefore a 
convenient species in which to begin a study 
of reptilian bulbospinal pathways. 
A question of particular interest is whether 
there are any basic, ancestral mechanisms of 
supraspinal motor control which are common 
to all vertebrates. An answer to this question 
will be sought by looking at the organization 
of descending pathways in the Tegu lizard 
in terms of their patterns of termination. 
Additionally, the origins of the different 
descending pathways and the organization of 
their target cells in the spinal cord will be 
considered. 
MATERIALS AND METHODS 
The Tegu lizards used in this study were 
identified as Tupinamhis nigropunctatus ac- 
cording to the key in Peters and Donoso- 
Barros (1970). The normal organization of 
the spinal cord (Cruce, 1978) was studied in 
animals which had not been subjected to 
experimental surgery. Spinal cords were sec- 
tioned in ajl three standard planes and 
stained with the Nissl method for cell bodies, 
the Heidenhain method for myelinated fibers, 
or the Golgi method for cell bodies, dendrites, 
and axons. 
Descending fiber projections were studied 
(Cruce, 1975; Cruce, et al., 1976) by per- 
forming lesions at the first spinal segment 
and observing anterograde degeneration of 
the severed fibers in the spinal cord using the 
Nauta or Fink-Heimer reduced silver stains 
(Ebbesson, 1970; Heimer, 1970). 
Brainstem cells giving rise to descending 
fiber projections were localized (Cruce, et al., 
1976) by performing lesions at the first 
spinal segment and observing retrograde 
chromatolysis of cells whose fibers had been 
severed. These brains were from the same 
animals used for studying descending fiber 
degeneration so that terminal patterns could 
be accurately correlated with cell groups of 
origin. 
RESULTS 
Spinal Cord Morphology 
Excellent reviews of the classical work on 
the reptilian spinal cord are available in the 
