116 
Cruce 
(Bodian, 1937) and raises the intriguing pos- 
sibility that different types of synaptic input 
might be confined to either the medial or the 
lateral dendrite. 
Terminal Fields of Descending Fibers 
Three striking differences can be noted 
when the pattern of descending fiber ter- 
minations seen in this lizard is compared to 
that described by Nyberg-Hansen (1969) in 
cat; (1) the absence of terminals in the 
dorsal horn, (2) the prevalence of terminals 
in the contralateral spinal gray without any 
associated descending fibers in the contra- 
lateral white matter, and (3) the sparseness 
of terminals in the lateral regions of the 
spinal gray. In the following paragraphs 
each of these points will be considered. 
The terminals in the cat dorsal horn 
(laminae IV, V) and a portion of those in 
the lateral spinal gray (V, VI, VII) are 
from fibers originating in cerebral cortex 
(Nyberg-Hansen, 1966; Petras, 1967). The 
absence of terminals in the lizard dorsal horn 
and their sparseness in the lateral gray 
matter is consistent with the view that a 
corticospinal pathway does not exist in rep- 
tiles (Ariens Kappers, et al., 1936; Goldby, 
1937 ; Goldby and Gamble, 1957 ; Kruger and 
Berkowitz, 1960) , although it should be noted 
that electrophysiological and neuroanatomi- 
cal techniques have not yet been used to 
conclusively demonstrate the presence or 
absence of this pathway. 
Mammalian descending fibers terminate 
predominantly on the side of the spinal cord 
where they descend; most decussations are 
located in the brainstem. However, some 
fibers decussate in the spinal cord at high 
levels to descend for a distance in all areas 
of the contralateral funiculi before terminat- 
ing in both lateral and medial regions of the 
spinal gray (Nyberg-Hansen, 1969). Other 
fibers appear to terminate in the segment 
where they decussate (Nyberg-Hansen, 1969 ; 
Martin and Dom, 1971), as in the lizard, 
although not with the same intensity. 
Since the number of degenerated fibers in 
the dorsolateral funiculus is roughly equal 
to the number seen in the ventromedial 
funiculus, it is surprising that the terminal 
degeneration following a dorsolateral lesion 
is so much less than that following a ventro- 
medial lesion. This difference is dramatic 
following a complete hemisection (Fig. 3) 
of the lizard spinal cord; degeneration in 
the lateral parts of the spinal gray is far 
less intense than that in the medial parts. 
Two possible explanations are; (1) The 
dorsolateral fibers have less extensive arbo- 
rizations near their endings, and (2) the 
dorsolateral fibers have fewer collaterals 
into the spinal gray. In mammals, fibers in 
the dorsolateral funiculus terminate in the 
lateral regions of spinal gray with about 
the same intensity as those in the ventro- 
medial funiculus terminate in the ventro- 
medial regions (Nyberg-Hansen, 1966). 
Mammalian rubrospinal fibers descend in 
the dorsolateral funiculus and project to the 
lateral parts of laminae V, VI, and VII 
(Kuypers, 1973; Petras, 1967; Nyberg-Han- 
sen and Brodal, 1964 ; and Martin and Dom, 
1970). The mammalian medullary reticulo- 
spinal pathway sends fibers in the lateral 
funiculus to project to lateral lamina VII 
while the pontine reticulospinal pathway 
descends ventromedially and projects to la- 
mina VIII and medial lamina VII (Kuypers, 
1973 ; Petras, 1967 ; Nyberg-Hansen, 1965 ; 
Martin and Dom, 1971). In lizards (Robin- 
son, 1969; Cruce et al., 1976) rubrospinal 
fibers descend in the dorsolateral pathway, 
medullary (my el encephalic) fibers descend 
ventrolaterally, and “pontine” (metence- 
phalic) fibers descend ventromedially (liz- 
ards have no true pons) . 
Supraspinal Effects on Motoneurons 
The terminal field of fibers descending in 
the medial longitudinal fasciculus (MLF) 
to the spinal cord is probably confined to 
the medial part of the ventral horn (laminae 
VIII and medial IX). A Number of brain- 
stem cell groups, particularly interstitial and 
reticular nuclei, are thought to contribute 
