THE NERVOUS SYSTEM. 83 



The action of the nerves is similar to reflex action, only 

 that an effort of will is needed to send an impulse from 

 the brain. It is by the help of the brain along this line 

 that an infinity of artificial reflexes or habits is acquired, 

 for which volition is needed in the beginning but which 

 are later done unconsciously. Herein lie the possibilities 

 of all education. 



The brain and spinal cord work together, the cord 

 acting as a medium between the brain, in which all the 

 higher psychical processes, such as will, thought, etc., 

 originate, and the muscular apparatus. The cord, 

 however, has some action entirely independent of the 

 brain, as is seen in reflex action. This action is entirely 

 involuntary, so that the cord is sometimes spoken of as 

 the seat of involuntary action, commonly called reflex 

 action. All unconscious acts are reflex acts, as when 

 the hand is drawn away from a hot iron. If an impulse 

 is sent along one of the sensory fibers, it enters the cord 

 through the posterior horn, where its nerve cell is found. 

 Then, through some connection between the nerve cell 

 of the sensory fiber and that of the motor fiber the im- 

 pulse is transmitted to the motor cell and another impulse 

 is sent out of the cord along the motor fiber of the nerve 

 to the muscle. One of the commonest reflexes is the 

 knee-jerk. Reflex action is important because the re- 

 flexes are interfered with, delayed, destroyed, or in- 

 creased in different diseases. The time normally re- 

 quired for a reflex act is very brief, that for the knee-jerk 

 being about three one-hundredths of a second. 



The nerves of the head, known as the cranial nerves, 

 arise from the brain, while the rest of the body is sup- 

 plied by the spinal nerves, which come off at intervals 

 from the spinal cord. The cranial nerves consist of 

 twelve pairs: (1) The olfactory or nerve of smell, (2) 

 the optic or nerve of sight, (3) the motor oculi, (4) the 

 patheticus, which controls the eye, (5) the trigeminus 

 or trifacial, a nerve of general sensation, motion, and 

 taste, (6) the abducens, a motor nerve, (7) the facial 



