THE BRAIN 295 



that part of the brain lying under the back of the skull 

 is called the occipital region. Each region of the brain 

 does a special work. 



531. Action of the cerebrum. The nerve centers may 

 act reflexively in response to sensory impulses, as the spinal 

 cord usually does; or automatically by originating their 

 own impulses, like the respiratory center in the medulla 

 and the nerve cells in the heart. The spinal cord, sym-. 

 pathetic system, medulla, and cerebellum all act in one or 

 the other of these ways, and without our being conscious 

 of their action. The cerebrum is the seat of the thinking 

 mind. It acts in an automatic way, but we may be con- 

 scious of any of its actions. It acts first by feeling sensa- 

 tions; second by sending orders for voluntary muscular 

 movements ; third, by thought. It does each kind of work 

 in a particular region of its surface. 



532. Sensory regions. Sensory impressions of which 

 we are conscious are sensations. Sensations of hearing, 

 smell, and taste are felt by the temporal region ; of sight 

 by the occipital region ; and of touch by the parietal region. 

 If either region is destroyed, the impressions going to that 

 area are no longer received, and the person is devoid of the 

 corresponding sense. Unless each impression reaches its 

 own region of the surface of the brain, it produces no sensa- 

 tion, although it may still reach reflex centers in the optic 

 tubercles, medulla, or cord, and give rise to reflex action. 



533. Memory. Impressions may be retained in the cells 

 and be recalled. These constitute memories. Our memo- 

 ries are complex stores of impressions in widely separated 

 parts of the brain. The sum of our different memories 

 constitutes a great part of our knowledge. 



Different regions of the brain are connected by nerve 

 fibers. So when one region recalls a memory, another 



