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HUMAN BIOLOGY 



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FlG. 103. — Sliowing a NEU- 

 RON, .-/, or nerve cell with 

 all its parts — dendrites, 

 cell body, and axon ; B, a 

 portion of a white fiber 

 highly magnified. (Jegi.) 



municatc with every other sub- 

 scriber, would need one thousand 

 wires running into his house ; all 

 together, there would have to be 

 several hundred thousand (to be 

 exact, 499,500) wires. With a cen- 

 tral office only one thousand are 

 needed. As a telephone system 

 has central offices, so the nervous 

 system has nerve centers. Nerve 

 centers contain nerve cells. Al- 

 though there are some subordinate 

 nerve centers in the spinal cord, 

 the greatest collection of nerve 

 centers in our bodies is in the skull, 

 and is called the brain. Fishes 

 were the lowest animals studied in 

 animal biology found to possess a 

 true brain. 



The nervous system, unlike a 

 telephone system, has other duties 

 besides allowing communication. 

 It enables us to think, and, after 

 reflection, to will and to act by con- 

 trolling the various cfrgans. 



The Units of which the Nervous 

 System is Constructed. — A nerve 

 cell with all its branches, or fibers, 

 is called a neuron (see Fig. 103); 

 some neuron branches are several 

 feet long. Neurons are the units 

 that compose the nervous system. 

 The living substance in cells is 



