Nervous Systems 841 



visual response is pupillary, whereas in fish and amphibia all vision may be 

 mediated bv the tectum. Dogs and monkeys have subcortical perception of 

 light and dark, but not of pattern.^^" 



The cortex is also distinguished by a certain equipotentiality of function. 

 Although there is point-for-point projection on the visual cortex (Fig. 140), 

 deficiencies in learned visual reactions occur in rats in proportion to the 

 amount of cortex removed, largely irrespective of location. ^^"' -^^ Removal 

 of a visual area from monkeys causes a loss of a learned visual habit but the 

 habit is readilv relearned, presumably by a new area."* Auditory learning in 

 dogs can occur in substitute areas, any of three regions on both sides being 

 required in varying amounts.^- When high intensities of electrical stimula- 

 tion and facilitation are used instead of threshold stimuli for mapping motor 

 areas, there appears to be more overlap of the areas eliciting movements of 

 different limbs.''"' Histological identification of different areas of cortex in 

 monkeys shows much variation among the individuals of a species, differ- 

 ences which may even be the basis for individual variation in behavior.^^'- 

 The histological localization of specific cortical areas is by no means as pre- 

 cise as was formerly believed.''" 



Mammals, as compared with birds and reptiles, show striking increase in 

 specificity of function of the neocortex, with lamination and the development 

 of cortico-spinal, particularly pyramidal, tracts. Motor areas are present even 

 in monotremes and marsupials, but localization is less precise, fewer kinds of 

 movement are elicited, and there is more overlap than in higher mammals. 

 The basic response to electrical stimulation of a motor area is a gross move- 

 ment; increase in the number of neurone lavers and increase in cortical 

 folding permit more discrete localization and more modifiability of response. 

 The evolution of the neocortex in mammals has involved an increase in 

 number of "computer circuits," a greater possible variety of combinations, 

 and a greater memory factor. 



Behavior mediated by the cortex differs from responses of lower centers 

 in consisting of delayed reactions and in showing considerable lability of re- 

 sponse. The interconnections of cerebral neurones are multiple. The true 

 picture of cortical function combines gross localization and labilitv of finer 

 units; it is doubtful that the mechanisms of cortical lability can be under- 

 stood by any of the experimental techniques now available. 



PERSISTENT PHENOMENA IN CENTRAL NERVOUS SYSTEMS 



Most of the behavior described in the preceding sections can be accounted 

 for by messages acting for brief periods over fixed anatomical pathways. 

 Nerve physiologists have concentrated on mechanisms of transmission, fa- 

 cilitation, and inhibition, which are transient, lasting at most a few hundred 

 milliseconds. Less understood is the capacity of the nervous system to retain 

 patterns of response for long times— seconds, days, and a lifetime. We know 

 virtually nothing of the physiological basis for complex "instinctive" behav- 

 ior, conditioning, learning, memory, and abstract reasoning. Tropistic be- 

 havior, direct reaction, rarely occurs in nature, where animals are subjected 

 to multiple stimuli and where reactions depend on past experience. 



Changes in excitability persisting for seconds in mammalian nerve cen- 

 ters have been described. The summation in the lumbar spinal cord associ- 



