272 Why Liv'mg 



inside a small cavity filled with fluid 

 (cerebrospinal fluid). The brain sub- 

 stance around the cavity is whitish in 

 color. Outside of the white matter lies 

 the region of the "gray matter" of the 

 brain, better known as the cortex. In all 

 the mammals the cortex is in folds or 

 ridges known as coiwolutiofis (kon-voh- 

 lew'shuns). If this cortex were spread 

 out smoothly, it would cover an aston- 

 ishingly large surface. 



The cerebellum, like the cerebrum, 

 has white matter mside, surrounded by 

 gray matter. In the medulla, however, 

 the gray matter is mixed irregularly with 

 the white matter. In the spinal cord the 

 gray matter forms a column inside the 

 white matter. This column of gray mat- 

 ter looks, in cross section, much like an 

 "H" or a butterfly with wings spread 

 out. 



Activities of the cerebrum. Through 

 study of diseased and accidentally injured 

 brains, we have learned much about how 

 the various parts of the brain work. Ex- 

 periments with animals, too, have con- 

 tributed much information, for there is 

 good reason to believe that the diff"erent 

 parts of the brain work much the same 

 in all vertebrates. Let us see what has 

 been learned by experiments with pi- 

 geons. In one common type of experi- 

 ment parts of the bird's brain are re- 

 moved, one after the other, and the 

 behavior of the bird is studied after it 

 has recovered from each operation. 

 When the cerebrum is removed the bird 

 seems, at first glance, to act like a normal 

 bird: it can still hop and move its wings; 

 it can still peck at food and swallow. 

 But closer observation shows that unless 

 its bill comes into contact with food the 



Things Behave As They Do unit v 



bird will not eat; it does not see or recog- 

 nize the food. It fails to initiate any ac- 

 tions. Only the original, inborn behavior 

 (reflex), remains unchanged. 



Let us see what this shows about the 

 cerebrum. The bird failed to see or rec- 

 ognize food. Evidently, while the eye is 

 the receptor, it is the cerebrum which in- 

 terprets the impulses that come from the 

 eye. Various other experiments all lead 

 to the conclusion that sensations depend 

 on the sense organs and the cerebral cor- 

 tex combined. The part of the cortex 

 receiving impulses from the sense organs 

 is called the sensory region. One part of 

 the human cortex is used in sight, the 

 part lying in the back of the head. An- 

 other part, the region on the side above 

 the inner ear, is used in hearing. Other 

 parts of the cortex are used in smelling 

 and tasting. Our sensations are all de- 

 pendent upon something that happens in 

 the cerebral cortex after it has received 

 impulses from the receptors. 



Secondly, while the bird retained its 

 inborn reflexes, such as pecking at its 

 food once the food touched the bill, it 

 is clear that initiation of impulses occurs 

 in the cerebrum. Here lie centers which 

 control the contraction of voluntary 

 muscles all over the body. These centers 

 are called motor areas because the activ- 

 ity here results in motion. Impulses from 

 the motor areas do not go to the various 

 muscle cells directly but are relayed 

 through other parts of the brain. Further- 

 more we have learned from these and 

 other experiments and observations on 

 human beings that memory, thought, 

 learning, will, and the emotions are asso- 

 ciated with the cerebrum. The well- 

 de\'eloped cerebrum, such as is found in 



