404 THE NERVES AND THE SENSE ORGANS 



The student may wonder how the afferent impulse that comes to the 

 central system can be " switched over" into the efferent neurons, and 

 sent back to the organs. The making of this connection is the work of 

 the associating neurons. If the proper associating neurons are in the 

 spinal cord, then the impulse will go no higher than the cord; it will be 

 turned over, at once, to the efferent neurons. For certain impulses 

 the associating neurons are in the bulb ; for others they are only in the 

 cerebrum. 



404. Voluntary Action. We have already learned 

 (cf. 355 and 398) that the organs of the body are moved 

 by muscles, and that some muscles are under the control 

 of the will (voluntary muscles), while other muscles are 

 involuntary, carrying out their functions whether we 

 want them to or not. All the muscular action that 

 comes from the use of the will gets its impulse in the 

 cerebrum (cf. 400). If we see a pencil lying on the 

 floor, the sight of the pencil acts as a stimulus to the 

 cerebrum and to the mind. If we want the pencil, the 

 cerebrum sends the proper impulses to the muscles; we 

 stoop, and pick up the pencil. But it is not necessary for 

 us to pick up the pencil at all; we may prefer to step upon 

 it instead. Thus the mind has the power of choosing 

 what stimuli it will send back to the muscles. 



405. Reflex Action. Reflex actions are involuntary 

 actions. They are called reflex, meaning "bent (or 

 turned) back," because the afferent impulses that start 

 them are turned back as efferent impulses. If the asso- 

 ciating neurons are below the cerebrum, we are not 

 conscious of either the stimulus or the response of the 

 organ (cf. 403). Of this sort of reflex actions are the 

 movements of the internal organs, such as the stomach, 



