Forebrain and Midbrain of Lizards 
53 
population with different receptive proper- 
ties. Thus each lamina receives visual input 
only from stimuli of special biological sig- 
nificance, These ganglion cells have been 
selected to convey information about only a 
fraction of the total visual environment, 
primarily relating to predator and prey 
objects. 
Comparable studies do not exist for rep- 
tiles. In geckos as in amphibians, tectal abla- 
tion or interruption of optic tract fibers pro- 
jecting to the optic tectum results in a loss 
of visually guided predator-prey behavior 
(unpublished observations). Similar losses 
in pattern discrimination have been reported 
in Podocnemis (Bass, Fritz and Northcutt, 
1973). While these studies implicate the 
tectum in visual pattern recognition and in- 
dicate that it is an essential pathway for 
predator-prey related behaviors, they do not 
provide information on the role of the tectum 
in such complex behaviors as layout of home 
range, defense of territories, possible intra- 
specific individual recognition, utilization of 
seasonal or spotty food resources, learned 
avoidance of specific predators, etc. Several 
of the behavioral reports presented in the 
present volume (for example, Jenssen and 
Auffenberg) clearly argue that such behav- 
iors do exist in lizards. 
While our present information regarding 
the neural circuitry of reptiles is far from 
complete, a bare skeleton of major organi- 
zational features can be assembled (Figs. 19, 
20). These features of reptilian neural or- 
ganization allow a number of hypotheses to 
be formulated regarding function and sug- 
gest many areas where much work remains 
to be done. 
Consideration of even simple motor acts in 
reptiles such as orientation to potential prey, 
agonistic gaping, head and trunk bobbing 
reveals that many muscles in distinctly dif- 
ferent parts of the body are innervated by 
both spinal and cranial components. Further- 
more, the movements of these muscles are 
Figure 20. Summary of experimentally determined descending pathways in reptiles. Most major brain cen- 
ters terminate within the reticular formation (here defined to include the red nucleus and vestibular nuclei 
which are usually considered derivatives of the reticular formation). Massive convergence of descending 
pathways onto the reticular formation suggests its possible role as a pattern generator in reptilian be- 
havior. The descending pathways of the dorsal ventricular ridge are poorly understood at present. C, cere- 
bellum; CN, cerebellar nuclei; CNC, cranial nerve motor column; DC, dorsal cortex; DVR, dorsal ventricu- 
lar ridge; H, hypothalamus; MC, medial cortex; OC, optic chiasm; OT, optic tectum; i2F, reticular forma- 
tion; iS, septal nuclei; SMC, somatic motor column of spinal cord; ST, striatum; T, torus semicircularis ; 
VT, ventral thalamus. 
