THE CENTRAL NERVE SYSTEM 353 



within the cells, energy is liberated and the nerve 

 impulse is conveyed by the efferent nerve to the muscle 

 and it contracts. 



In most reflex actions there is and must be a more 

 complex arrangement to account for the varied move- 

 ments and functions of the different structures of the 

 body which are taking place in response to external 

 and internal stimuli. These complex reflexes are 

 due to the nerve centres in the cord communicating 

 by means of axones and dendrites of other cells with 

 other higher centres at different levels of the cord, 

 not only on the same but the opposite side; and a 

 still more complex arrangement is produced, due to 

 the fact that the centres in the medulla oblongata are 

 in connection with the spinal centres by pathways 

 of nerve fibers. 



Thus reflex actions can be carried on without the 

 individual being conscious thereof, or by the presence 

 of the associated neurones other centres convey sensa- 

 tions to the brain, of which we are conscious, and the 

 return impulse can excite a voluntary movement. 



Reflex actions are controlled by centres in the brain 

 (medulla) which transmit impulses to the spinal 

 centres which either decrease or inhibit, either increase 

 or accelerate their activity, thus regulating the recep- 

 tion of and response to nerve impulses by these 

 centres which exert a controlling influence based on 

 the needs of the physiologic functions of the human 

 body. 



Association Conduction. The spinal cord is divided 

 into segments which have a controlling influence 

 over the physiologic functions of certain parts of the 

 body, as the arm, leg, etc. These segments to properly 

 work together and receive impulses from other reflex 

 centres at different levels in the cord, which control 

 movements in response to stimuli from other portions 

 of the body, are held in communication by pathways 

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