848 Comparative Animal Physiology 



the intergrative basis is less so. In each of the most specialized animal groups 

 are regions of the "brain" which are not sensory centers and which may medi- 

 ate no direct motor response when stimulated— the verticalis complex of ceph- 

 alopods, "association areas" of the cerebral cortex of mammals, and probably 

 the corpora pedunculata of arthropods. As cephalization proceeds there is in- 

 creasing control of one level of the nervous system by another level, the 

 cephalic levels usuallv inhibiting and regulating the lower motor levels. 



All integrative nerAC centers which have been examined show "spontan- 

 eous" electrical activity, which is not shown in distributing or sensory cen- 

 ters. The integrative centers — earthworm ventral ganglia, goldfish tectum, 

 and mammalian cortex — show true conditioning. 



Both phylogenetically and ontogenetically two trends in development of 

 nervous systems are evident: (1) from mass reactions to restricted compli- 

 cated behavior patterns, and (2) from fixed rigidity of response to lability of 

 response. The movements of most worms, of lower fish, and of early embrvos 

 of all animals are gross total patterns, whereas restricted movements appear 

 as locomotor appendages develop. The nervous system constantly integrates 

 the behavior of each animal as a whole. Neural rigidity implies a simple 

 stimulus-response system, lability requires integrative mechanisms. In the ver- 

 tebrate series the primitive neuropile may show some lability, but as the series 

 progresses the associative regions are pushed cephalad and increase greatly 

 in amount. New regions of the brain take o\'er functions ser\'ed by lower 

 regions in less specialized and less versatile vertebrates. 



In a survey of comparative neurophysiology one is impressed by how 

 much is known about transmission in axons and across synapses and how 

 little is known about central nervous phenomena that persist longer than a 

 few milliseconds. There are many examples of behavior for which no physio- 

 logical mechanism is known. Animals such as arthropods immediately 

 alter leg coordination when one appendage is interfered with; this is plas- 

 ticity of the nervous system. In other animals, particularly in amphibians, the 

 whole spinal cord contains some means of "tuning" a specific muscle-reson- 

 ance of the nervous system. Locomotion is more than a sequence of chain 

 reHexes. The specific behavior associated with feeding, mating, nest-building, 

 care of young, etc., in such animals as insects, fishes, birds, and even poly- 

 chaete worms, is unlearned and very complex. Modifiability of response by 

 repeated or by associated sensgry stimulation is varied, and no adequate 

 classification of types of "conditioning" exists. Finally learning, abstract rea- 

 soning, and in man the evolution of speech are neural phenomena whose 

 nature remains for the future to explain. 



REFERENCES 



1. Abbie, a. a., Ajistral. ]. Exper. Biol. & Med. Sci. 16:143-152 (1938). E.xcitable 

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2. Abbie, A. A., ]. Comp. Neurol. 72:469-485 (1940). Excitable cortex of mar- 

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3. Ades, H. W., /. Netirophysiol. 2:415-424 (1944). Midbrain auditor^' mechan- 

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4. Ades, H. W., /. Neuropath. & Exper. Neurol. 5:60-65 (1946). Cortical function 

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5. Ades, H. W., and Feeder, R. E., J. Neurol. 8:464-470 (1945). Acoustic pro- 

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