Forebrain and Midbrain of Lizards 
39 
visual pathways to the telencephalon and 
that these pathways may subserve very dif- 
ferent visual functions. 
In addition to the retino-recipient nuclei 
that have been described, the dorsal thalamus 
of lizards is formed by a number of nuclei. 
These nuclei are : nucleus dorsolateralis, 
nucleus dorsomedialis, nucleus medialis, nu- 
cleus rotundus, and nucleus medialis pos- 
terior (Figs. 10-13). 
Nucleus dorsolateralis and nucleus dorso- 
medialis form the rostral pole of the dorsal 
thalamus and extend to a mid-thalamic level 
where nucleus rotundus first appears (Figs. 
10, 12). Nucleus dorsolateralis does not ex- 
tend beyond nucleus rotundus. However, 
nucleus dorsomedialis continues more cau- 
dally and is continuous with the pars plicata 
of nucleus lentiformis thalami. Nucleus dor- 
solateralis gives rise to the most rostral 
component of the dorsal peduncle of the 
lateral forebrain bundle (Figs. 10, 12). Re- 
cently Distel and Ebbesson (1975, and per- 
sonal communication) have reported on the 
efferent projections of this nucleus in 
Varanus. Lesions of nucleus dorsolateralis 
give rise to degenerating pathways that 
course in both the lateral and medial fore- 
brain bundles and enter the ipsilateral 
telencephalon. These pathways appear to 
terminate in nucleus accumbens, septum, 
striatum, dorsolateral cortex, and pallial 
thickening. Because nucleus dorsolateralis 
lies just medial and ventral to the retino- 
recipient thalamic nuclei, there is a strong 
possibility that one or more of the telen- 
cephalic areas believed to receive fibers from 
nucleus dorsolateralis may, in fact, arise 
from these visual nuclei whose axons were 
interrupted by lesions of nucleus dorsola- 
teralis. Due to the complexity of the rostral 
dorsal thalamus in lizards, additional studies 
using horseradish peroxidase will greatly aid 
the analysis of this region. On the basis of 
horseradish peroxidase studies. Parent and 
Boucher (1975) have suggested that nucleus 
dorsolateralis and nucleus dorsomedialis 
project to the striatum in Chrysemys. At 
present, the afferent connections of nucleus 
dorsolateralis and nucleus dorsomedialis are 
unknown. Most workers have assumed that 
these nuclei in lizards are homologous to the 
anterior thalamic nuclei of mammals. If so, 
they would be expected to receive a projec- 
tion from the caudal hypothalamus — an- 
other reason why hypothalamic studies in 
reptiles are greatly needed. 
Nucleus rotundus is usually the largest 
single thalamic nucleus in lizards. It is 
capped rostrally by the dorsolateral and dor- 
somedial thalamic nuclei and occupies the 
greater part of the mid-thalamic zone, being 
replaced at more caudal levels by nucleus 
medialis posterior (Fig. 11). The optic tec- 
tum gives rise to the major afferents (both 
ipsilateral and contralateral pathways exist) 
to nucleus rotundus, which in turn projects 
upon the ipsilateral striatum and a dorso- 
lateral division of the dorsal ventricular 
ridge (Hall and Ebner, 1970; Pritz, 1973, 
1975; Distel and Ebbesson, 1975). 
Recently Benowitz and Karten (1975) 
have reported that nucleus rotundus in birds 
also receives afferents from pretectal nuclei 
and a thalamic nucleus (nucleus reticularis 
thalami) as well as from the tectum. These 
results strongly suggest that similar path- 
ways exist in reptiles since, to date, the 
majority of discovered avian neural path- 
ways have also been found in reptiles. 
In many lizards nucleus rotundus is sur- 
rounded by an annulus, or belt of neurons, 
called the perirotundal belt (Figs. 10, 13). 
At present nothing is known about the con- 
nections of these cells in lizards. However, 
findings with horseradish peroxidase have 
led Parent and Boucher (1975) to believe 
that these cells project upon the striatum in 
Chrysemys. 
Nucleus medialis is located at mid-thalamic 
levels, ventral or medial to nucleus rotundus. 
In varanids and Xantusia, as well as in some 
turtles and crocodiles, it fuses across the 
midline and is then termed nucleus reuniens. 
Nucleus medialis is an auditory relay nucleus 
that receives afferents from the midbrain 
auditory area, torus semicircularis, and pro- 
jects to the ipsilateral striatum and to a 
medial division of the dorsal ventricular 
