RESPONSIVENESS IN VERTEBRATES 



tions a bell is rung whenever food is given to an animal. After a number of 

 such experiments the mere ringing of the bell, without the sight of food, will 

 result in the secretion of saliva. In this way a stimulus that originally had 

 no effect upon the salivary glands has become as.sociated with one to which 

 the glands respond. As a result of this as,sociation the previously indifferent 

 stimulus of the ringing bell becomes effective in producing the reaction of the 

 salivary glands; a conditioned reflex has been established. Experiments and 

 analysis of conditioned reflexes make it clear that many human reactions are 

 the result of such correlations. Our responses to warning colors, signals, and 

 nationally used signs and symbols are in the nature of conditioned reflexes. 

 The same explanation holds for many more subtle and less well-understood 

 adjustments. 



Localization of Function in the Nervous System. The basic mech- 

 anisms of excitation and conduction are shared by all parts of the nervous 

 system. Different regions of the nervous system do, however, have certain 

 distinctive functions. 



The general function of the peripheral nervous system is to conduct nerve 

 impulses to and from the central nervous system. In regard to the spinal 

 nerves, it has been pointed out that processes of afferent neurons enter the 

 spinal cord over the dorsal roots of spinal nerves, and the processes of efferent 

 neurons pass out along the ventral roots. The spinal nerves are called mixed 

 nerves and may be considered to represent the primitive condition of nerve 

 trunks. Certain of the cranial nerves, as the third or oculomotor, also carry 

 processes of both sensory and motor neurons. Other cranial nerves carry 

 processes of but one type of neuron. The eighth cranial or auditory nerve is 

 made up entirely of processes of afferent neurons from the ear; the eleventh 

 and twelfth cranial nerves, found in the higher vertebrates, contain processes 

 of only efferent neurons. Finally, the autonomic nerves are entirely efferent 

 and constitute the final common paths to glands and to non-striated muscles 

 of the blood vessels and viscera. 



As has been repeatedly implied in the discussion of the reflex arc, the gen- 

 eral function of the central nervous system is the adjustment of incoming to 

 outgoing impulses. It is in the central system that afferent neurons have 

 synapses with adjustor neurons, and these in turn with efferent neurons. The 

 multiplicity of connections thus made possible furnishes the most important 

 part of the mechanism of integration. 



Adjustor neurons in the spinal cord are related to the simpler and less 

 complicated of the reflex arcs. In the scratch reflex, for instance, adjustor 

 neurons carry the impulse posteriorly in the spinal cord or conduct it from 

 side to side. Impulses entering the cord over spinal nerves can also pass 

 anteriorly to the medulla, cerebellum, and diencephalon. The cytosomes of 

 these adjustor neurons are located in the gray matter of the spinal cord; 

 their processes, over which impulses are conducted along the cord, are to be 

 found in the white matter. The white matter also contains groups of nerve 

 processes which arise from the cell bodies of adjustor neurons located in 



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