The Medial Lemniscus in Lizards 
93 
Figure 1. Site of lesion in dorsal column nuclei (left) and course of resulting degeneration (right). 
central nucleus of the torus semicircularis, 
and (4) the central gray area (Fig. 5). 
Slightly further rostrally (Fig. 6), terminal 
degeneration is also found in the medial 
corner of the tectum. This zone is near the 
mesencephalic nucleus of the trigeminal 
nerve and involves all layers deep to the 
stratum griseum superficiale. At this level 
(Figs. 6-7), some terminal degeneration is 
found in a cell group just dorsal to the 
ectomammillary nucleus; this area may be 
homologous to the substantia nigra of mam- 
mals. The few remaining degenerating axons 
can be traced into the ventral thalamus 
where most of the degeneration observed is 
in the so-called ventrolateral nucleus (Figs. 
7-10. This latter area is here labeled the 
ventrobasal complex (Vb) because of its 
striking similarity to the mammalian cell 
groups of that name. 
DISCUSSION 
The new findings reported here, coupled 
with earlier descriptions of ascending spinal 
projections (Ebbesson, 1966, 1967 ; Ebbes- 
son and Goodman, 1976), provide clear evi- 
dence that the somatosensory pathways in 
lizards are very similar to those described 
in mammals (Mehler, 1957, 1969; Mehler 
et al., 1960; Schroeder and Jane, 1971; and 
others), amphibians (Ebbesson, 1976), elas- 
mobranches (Ebbesson, 1972), and birds 
(Karten, 1963). The differences in spinal 
projections in the various taxonomic groups 
relate primarily to the relative quantity of 
fibers in a given pathway and the degree of 
differentiation of the various related nuclear 
groups. It is clear, for example, that the size 
and differentiation of the dorsal column sys- 
tem and the spinocerebellar systems are 
closely correlated with the degree of limb 
development (Ebbesson, 1969; Johnson, et 
