"SPl^ 



Effector 

 branch - 



SympathJetic 

 ganglia 



Receptor 

 branch 



,•1 



THE MAIN NERVE AXIS IN VERTEBRATES 



Among bilaterally symmetrical animals there is a main nerve axis with side branches 

 which lead to the skin and special sensory structures, and to muscles. Many stimuli 

 and many reactions of the organism may be completed within a narrow sector of 

 the body; but the cord carries vast numbers of connecting nerve fibers which relate 

 all parts of the body to all the other parts 



These neurons connecting different "levels" of the nervous system help 

 us to understand some of the more complex movements that appear to be 

 just as automatic as simple reflexes involving only the parts of a segment. 

 Let us suppose that one smells a strange odor. If it produces any impression 

 at all, one is likely to turn toward it — or away from it. In any case, the 

 muscles used may include those of the neck and shoulders and even those 

 of the trunk and legs. 



The Brain In all vertebrate animals the front end of the central nerv- 

 ous system is enlarged into a mass of neurons, connective tissue and blood 

 vessels, together constituting the brain (see illustration, p. 281). The average 

 weight of the brain of adult males in western Europe is about 1400 grams; 

 and of that of the females, about 100 grams less. Many human brains weigh 

 from 1500 to 1800 grams. With two exceptions, man's is the largest brain 

 among the inhabitants of the world. The true whale has a brain of 6700 

 grams; and the brain of the Indian elephant attains over 5400 grams. In 

 relation to the size of the body, however, man's brain is much greater than 



278 



