92 
Ebbesson 
Jane, 1971), indicating that these basic sys- 
tems evolved early in vertebrate evolution. 
The spinoreticular, spinovestibular, and 
spinocerebellar projections in the tegu lizard 
are strikingly similar to those described in 
mammals (Ebbesson, 1966, 1967 ; Ebbesson 
and Goodman, 1976) and will not be re- 
viewed here. We will instead limit ourselves 
to the finding of a medial lemniscus in two 
lizard species. 
The dorsal column system is an important 
component of ascending spinal systems, pri- 
marily reflecting the degree of limb develop- 
ment (Ebbesson, 1969). Although the gracile 
and cuneate nuclei have been described in a 
large number of nonmammalian species, the 
destination of their axons has remained a 
puzzle because it has not been possible to 
trace them for any distance in any non- 
mammal (Ariens Kappers et al., 1936, 1967). 
Our investigations with the new, more sen- 
sitive Fink-Heimer modifications of the 
Nauta stain were designed to fill this void. 
The projections of the dorsal column nuclei 
in the monitor lizard described below are 
identical to earlier observations in the tegu 
lizard (Ebbesson, unpublished observations). 
MATERIALS AND METHODS 
Six adult specimens of the monitor lizard 
(Varanus bengalensis) were anesthetized 
with sodium pentobarbital (Nembutal) (25 
mg/kg), and small portions of the dorsal 
column nuclei were removed with suction. 
The animals were maintained postoperatively 
at 30°-35° C, and sacrificed after 20-30 
days by intracardial perfusion with 10 per- 
cent formalin in water. The brains were 
removed and stored in 10 percent formalin 
under refrigeration for 1-10 weeks before 
embedding in egg yolk (Ebbesson, 1970a) 
and sectioning transversely at 33 m/i on the 
freezing microtome. At least one section per 
0.2 mm was processed according to a modifi- 
cation (Method 7 in Ebbesson, 1970a) of the 
Fink-Heimer (1967) technique for the selec- 
tive silver impregnation of degenerating 
axons and terminals. Additional sections 
were processed according to a modified Nauta 
procedure (Method 6 in Ebbesson, 1970a). 
The degeneration pattern of each section was 
charted on a photomacrograph (Ebbesson 
and Rubinson, 1971) of the adjacent section 
stained with the Nissl method (Fernstrom, 
1958). In the chartings (Figs. 1-10), dots 
and interrupted lines represent axonal de- 
generation, and open circles represent ter- 
minal degeneration as defined by Ebbesson 
(1970a). 
RESULTS 
Case 146 was chosen to illustrate the ex- 
perimental material since the findings in this 
case were easy to define and agreed with the 
findings in the other cases. The animal had 
24 days postoperative survival, and the lesion 
was restricted to the dorsal column nuclei. 
Axons leaving the dorsal column nuclei 
collect on the contralateral side as the medial 
lemniscus. This fiber tract is composed of 
very thin axons that form a narrow band 
in the ventral tegmentum (Figs. 1-2). Their 
small size and small number probably ac- 
count for the difficulty in tracing these axons 
when less sensitive methods are used (Ariens 
Kappers et al., 1936). 
At medullary levels, a few of the lateral 
fibers of the medial lemniscus appear to 
issue preterminal axons to the reticular 
formation (Figs. 2-3) and to the dorsal part 
of the facial nucleus (Fig. 2). At the level 
of the nucleus reticularis medius (RM), 
terminal degeneration is found in a poorly 
defined nucleus in the ventromedial tegmen- 
tum. This area is tentatively designated the 
inferior olive (01? in Figs. 3-4) since such 
a cell group in mammals receives an input 
from the dorsal column nuclei. At these 
levels, a lateral component of the medial 
lemniscus separates from the main tract 
and, coursing in a dorsal direction, reaches 
the torus semicircularis and the optic tec- 
tum. A few fibers enter and terminate in 
(1) the stratum griseum centrale of the 
optic tectum, (2) the nucleus intercollic- 
ularis, (3) the ventrolateral aspect of the 
