24 
Northcutt 
1970), Caiman (Fritz, 1973, 1975), and 
Varamis (Distel and Ebbesson, 1975), ap- 
proximately the lateral one- third (Fig. 4, 
area C) of DVR is the target of nucleus ro- 
tundus. This nucleus is a dorsal thalamic 
nucleus that receives projections from the 
optic tectum (Hall and Ebner, 1970; Butler 
and Northcutt, 1971a ; Braford, 1972 ; Foster 
and Hall, 1975). Thus, the lateral anterior 
division of the DVR receives visual informa- 
tion via a retino-tecto-rotundal circuit. 
The medial part of the anterior DVR 
(Fig. 4, area A) is the target of an ascend- 
ing auditory pathway (Weisbach and 
Schwartzkopff, 1967; Foster, 1974; Fritz, 
1974a, 19746; Distel and Ebbesson, 1975). 
In Caiman and Iguana, nucleus reuniens, a 
dorsal thalamic nucleus, receives projections 
from the torus semicircularis (Foster, 1974; 
Fritz, 1974a). The torus has been shown to 
receive auditory information from lower 
brainstem nuclei by both physiological 
(Manley, 1971) and experimental neuroana- 
tomical (Foster, 1974) methods, and should 
be considered homologous to the inferior col- 
liculus of mammals. Nucleus reuniens pro- 
jects to the medial DVR in Caiman (Fritz, 
19746), in Iguana (Foster and Feele, 1975), 
and in Varanus (Distel and Ebbesson, 1975). 
The auditory area represents the most 
caudal continuation of the anterior division 
of the DVR and can be seen to extend far 
caudally in the reptiles that have been ex- 
amined. 
A third sensory projection may be present 
in many reptiles. Lesions of the cervical spi- 
nal cord in Caiman (Northcutt and Bra- 
ford, unpublished observations) and in 
Tupinambis (Ebbesson, this conference) re- 
veal an ascending pathway that terminates, 
in part, in a dorsal thalamic nucleus located 
just caudal to nucleus rotundus. This caudal 
thalamic nucleus, nucleus medialis posterior 
of Huber and Crosby (1926), projects to the 
central part (Fig. 4, area B) of the anterior 
division of the DVR in Caiman (unpub- 
lished observations). At present the type of 
sensory information carried by this ascend- 
ing spinal pathway is unknown, but it is 
likely of a somatic sensory nature. A similar 
pathway has been reported to exist in the 
DVR of pigeons (Delius and Bennetto, 
1972). This central region of the DVR in 
birds is known to respond to somatosensory 
stimulation (Erulkar, 1955). 
A fourth high density SDH region can be 
identified on the lateral boundary of the 
DVR (area D, Figs. 2,3,4). This region 
might be considered a part of the ridge, but, 
because of its dorsal continuity with the 
lateral edge of the dorsal cortex, it could 
equally be considered part of the dorsal cor- 
tex. Its high concentration of SDH suggests 
that this region may be the target of yet 
another undescribed ascending sensory path- 
way. In this context, it is particularly inter- 
esting to note that an ascending trigeminal 
pathway has yet to be described to the telen- 
cephalon of reptiles. 
In birds, lesions of the main sensory 
trigeminal nucleus reveal a pathway that 
projects directly to a cell population located 
in the rostral basal telencephalon and termed 
nucleus basalis (Wallenberg, 1903; Cohen 
and Karten, 1974). A similar projection is 
observed in lizards and caimans following 
isthmal lesions (unpublished observations), 
and in these reptiles the comparable telen- 
cephalic nucleus is termed the nucleus of the 
lateral olfactory tract (Fig. 3) . This nucleus 
has been clearly misnamed as it does not 
receive an olfactory projection, but an as- 
cending isthmal projection, probably tri- 
geminal in nature. In birds, nucleus basalis 
projects to a cell population located on the 
lateral edge of the dorsal ventricular ridge 
(Zeier and Karten, 1971), which suggests 
that the “lateral olfactory nucleus” of rep- 
tiles may similarly project to area D located 
on the lateral edge of the DVR. 
The posterior division of the DVR can be 
characterized by lower levels of SDH, and, 
with the exception of the auditory ridge 
target which can be traced caudally into this 
region, does not appear to receive discrete 
thalamic projections in Iguana (Butler, per- 
sonal communication) or in Caiman (North- 
cutt and Braford, unpublished observations). 
In gekkonids, nucleus sphericus is re- 
stricted to the centromedial and caudal quad- 
