NERVOUS AND ENDOCRINE CONTROL I37 



cord) is itself a reflex center. All impulses are routed through it on their 

 way from receptor to effector. However, within this trunk there are 

 specialized areas which have superior powers of discrimination. These 

 might be compared to district, as opposed to local, telephone centrals. In 

 axially organized animals one of these superior reflex centers is always 

 located at the forward end of the main nerve trunk, in the head, where it 

 is in close touch with the speciahzed sense organs also located there. In 

 vertebrates this forward reflex center, the brain, dominates the other 

 reflex centers. To continue the telephone simile, the brain is a sort of super- 

 central which leaves routine business to the district centrals in the spinal 

 cord and elsewhere but which has forwarded to it all calls which are of 

 uncertain significance or which seem to require special action. 



The dominance of the brain over the other reflex centers was much 

 less marked in the early vertebrates than in the later ones. In some of the 

 dinosaurs, for example, the brain was actually smaller than the reflex center 

 at the rear end of the body. One of the most important features of verte- 

 brate evolution has been the increase in brain size relative both to the size 

 of the body and to the size of the other reflex centers. Coupled with this 

 there has been a steady increase in complexity of brain structure and in 

 specialization of function within the brain. 



In the lower vertebrates the brain functions mainly in the direct re- 

 ception of stimuli from the sense organs and in making automatic ad- 

 justments to these stimuli. At the amphibian level a new division of the 

 brain appears, the cerebrum. This specializes in more complex and selec- 

 tive reactions. As we come up the evolutionary scale, the cerebrum in- 

 creases in size in relation to the other parts of the brain and more and more 

 takes over the function of directing the individual. In primates and espe- 

 cially in man it quite overshadows the rest of the brain and takes care of 

 the organism's activities, with the exception of a few simple necessary 

 ones such as breathing, swallowing, and changing the size of the pupil of 

 the eye. 



The cerebrum is made up of an enormous number of neurons set in a 

 bed of connective tissue. There are at least 10,000,000,000 of these in the 

 brain of a normal human being. Each neuron is separated from its neigh- 

 bors by synapses. The paths of impulses through this maze of neurons and 

 synapses are not organized at birth but are established by the process of 

 path-wearing already described. Every time an impulse passes through 

 the cerebrum on its way from receptor to effector a large number of 

 neurons and synapses are involved and there is a change of some sort in 

 the cerebral structure. These changes are the structural basis of memory 

 and habit in the individual. The cerebrum is a specialized organ for learn- 

 ing and also for those higher forms of selection and integration of stimuh 

 which we call thought. 



The nervous system is the foundation of behavior, and, as far as we can 



