Behavior and Neurology of Lizards 
N. Greenbersr and P. D. MacLiean, eds. 
NIMH, 1978. 
Forebrain Connections in Lizards and the Evolution of 
Sensory Systems 
Ann B. Butler 
Department of Anatomy 
Georgetown University 
SUMMARY. Recent experimental studies have contributed new insights into the organization 
of sensory systems in nonmammalian vertebrate brains. Many workers have discarded the classi- 
cal view that the telencephalons of ancestral vertebrate species were dominated by olfactory in- 
put. The dorsal ventricular ridge of reptiles and birds, once regarded as the homolog of the 
mammalian basal ganglia, has been shown to be the target of ascending sensory system projec- 
tions, corresponding to similar systems which project to neocortex in mammals. Ascending pro- 
jections from the brainstem also terminate in the dorsomedial and dorsal cortical regions. While 
recent studies have supported the classical views that in reptiles the medial cortical wall cor- 
responds to parts of the mammalian hippocampal formation and the dorsal cortex to neocortex, 
the relationships of these regions in reptiles to each other and their corresponding structures in 
mammalian brains may be more complex than previously realized. In the lizard, Gekko, the 
medial wall of the hemisphere and the entire dorsal cortex have interhemispheric commissural 
connections via the hippocampal commissure. The evolutionary implications of these findings re- 
garding the mammalian neocortex and corpus callosum are discussed. 
INTRODUCTION 
Renewed interest in the anatomy of the 
neuroanatomical organization of nonmam- 
malian vertebrate brains has resulted from 
recent advances in histological methods for 
tracing connections, particularly the devel- 
opment of the Nauta silver impregnation 
technique and its modifications, and of intra- 
axonal transport methods. Given these tools, 
and those employed in electrophysiological 
and behavioral studies, a number of recent 
findings have led to a revision of some of our 
basic concepts of brain evolution and have 
thus stimulated further inquiries into this 
area. It has now also been recognized that the 
great range of variation in the morphology 
of nonmammalian vertebrate brains, when 
studied in the light of the differences in 
behavior, adaptive zone, electrophysiological 
proporties, etc., can yield a number of in- 
sights into the relationship of neuronal 
morphology and function. The present paper 
will review the anatomy of the pallial re- 
gions of the telencephalon in lizards in rela- 
tion to our present understanding of the 
evolution of sensory systems. 
ANATOMY OF THE LACERTILIAN 
PALLIAL REGIONS 
Cross sections through the telencephalons 
of two lizards, Gekko gecko and Iguana 
iguana, are shown in Figure 1. These two 
species will serve to demonstrate the range 
of variation in the morphology of the fore- 
brain among lizards (see Northcutt, this 
conference). It can be seen in both species 
that the cortical pallium can be divided into 
three major parts: medial, dorsal, and 
lateral. Each of these regions in turn can be 
subdivided. While relatively few detailed 
cytoarchitectonic studies have been done on 
reptilian telencephalons, some data regarding 
the distribution of cell types are available for 
a few species. 
Lateral wall. The lateral wall can be di- 
vided into lateral cortex pars dorsalis (LCd) 
and lateral cortex pars ventralis (LCv) on 
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