498 ELEMENTARY BIOLOGY 



514. Structure of the brain. In all vertebrate animals the 

 front end of the central nervous system is enlarged into a mass 

 of neurons, connective tissue, and blood vessels constituting the 

 brain (see Fig. 259). In man the brain is not only a larger 

 part of the whole body than it is in any other animal, but it is 

 absolutely the largest brain, excepting only that of some of the 

 larger elephants. 



The cortex, or "bark," of the cerebrum consists of nerve 

 cells. In mammals this gray layer is very much wrinkled, so 

 that there is relatively more surface than in lower vertebrates. It 

 seems that the extent of the convolution is related to the num- 

 bers of cells and to the complexity of their connections. The 

 white part of the brain consists of connecting fibers, or axons. 



On the ventral surface of the brain are many connecting 

 nerves, containing efferent and afferent fibers (see pp. 220-223). 

 The hind-brain and the mid-brain have to do with reflexes and 

 automatic movements of various kinds. In the cerebrum, nerve 

 action is connected with consciousness and voluntary movements. 



The activities of the heart, the digestive system, the breathing 

 apparatus, etc. may go on indefinitely without being influenced 

 in any way by what happens in the cerebrum, and without pro- 

 ducing any effect upon the cerebrum (except to keep it sup- 

 plied with blood). Many of our activities and movements are 

 unrelated to the cerebrum ; but every thought, every conscious 

 desire, and every deliberate or purposeful action depends upon 

 impulses starting from the gray matter in the brain or leading 

 to the gray matter. 



Experimental studies upon various mammals, and the experiences 

 with the diseased or injured brains of human beings, have established 

 the fact that each portion of the cerebral cortex is concerned with 

 specific feelings, ideas, or movements. In the diagram in Fig. 260 

 are indicated some of the localizations of brain function that have 

 been determined in these studies. 



The special study of the activities of the cerebrum, as they show 

 themselves in thinking, feeling, willing, is called psychology. 



