HUMAN STRUCTURE AND LIFE-ACTIVITIES 511 



The above are simple examples of unconscious reflex 

 actions which are constantly occurring. We learn to do a 

 large number of things reflexly, in addition to the funda- 

 mental processes such as breathing, heart-beat, digestion, 

 etc., which naturally and necessarily are subject to reflex 

 control. In walking, playing musical instruments, using 

 various tools, etc., we learn by long practice to act more or 

 less reflexly or automatically, and with little or no exercise 

 of the will (conscious control). 



439. Conscious or Voluntary Action. Touching* a hot 

 stove used above as an illustration of reflex action also 

 affords a case of conscious action. Soon after the hand is 

 jerked away by reflex action, one becomes conscious of being 

 burned. Obviously, there are nerve-fibers for transmitting 

 the sensory impulse from the tip of the burned finger to the 

 brain. There a conscious action may occur, and it may be 

 reasoned that it is dangerous to keep the hand anywhere 

 near a hot stove, and so on, with the result that it is decided 

 or willed to take the hand far away. This is accomplished 

 by a conscious motor impulse from the brain to the muscles 

 of the arm, causing them to move as the will dictates. 



Moreover, the brain may also control the direction of the 

 movement; that is, it may coordinate the contraction of 

 the various muscles. A splendid example of such coordinated 

 voluntary or conscious control of muscles is that of baseball 

 pitchers who can will to throw a ball to a given place and 

 at the same time consciously control the contraction of the 

 muscles of the arm so that the ball will be given the twirling 

 or curving motion so much desired by experts in ball-playing. 

 Learning to do this by long practice means training the nerve- 

 cells in the brain so that they will come to control the muscu- 

 lar contractions and so cause the desired muscular movements. 



440. Spinal Cord and its Nerves. The human spinal cord 

 is usually described as lying in a cavity in the backbone 

 (vertebral column). Examination of some of the segments of 



