158 HUMAN BIOLOGY 



ing the so-called message. The principal functions of the brain 

 may for convenience be divided into (1) reflex activities, (2) con- 

 scious activities, and (3) automatic activities or habits. 



234. Reflex activities. — To illustrate the reflex action of the 

 brain suppose we inhale some pepper ; a message goes up the nerves 

 to the cells in the nerve centers. This message is then reflected or 

 switched off to cells which send impulses down the nerves that 

 control the muscles of the chest. We then sneeze, and thus get rid 

 of the pepper. Coughing, winking, blushing, the flow of saliva at 

 the sight of savory food, — these are but a few of the reflex activites 

 carried on by the brain. 



235. Conscious activities. — As long as we keep awake, countless 

 nerve impulses keep pouring into our brains. When the cells of the 

 gray matter receive these impressions, we usually become conscious 

 that we are seeing, smelling, hearing, tasting, or feehng. These 

 sensations are more or less lasting, too, for we can recall distinctly the 

 appearance of objects that we saw yesterday, or even years ago, and 

 we can hear again, as it were, the sounds we have heard in the past. 

 In some unknown way these impressions are stored away in the 

 protoplasm of the brain, and constitute our memory. 



Another power of which we are conscious is the ability to direct the 

 movements of the body. We can rise from a seat, walk about, talk 

 or change the expression of our faces as we will. 



236. Habitual activities. — If we can remember the time when we 

 learned to write, we recall that each letter was traced laboriously 

 by a conscious effort of our brains to guide the muscles of our fingers. 

 Writing, in our early years, belonged to the group of our conscious 

 activities. But as time went on, less and less of our attention was 

 needed for this mechanical process, until now our fingers seem to 

 move of themselves. Walking, bicycle riding, swimming, playing 

 the piano, convejdng the food to our mouths — none of these activi- 

 ties require our attention. We have made these movements so 

 many times that they have become awtomatic. In other words, the 

 conscious part of our brains has trained other nerve centers to 



