^74 



Why Living Things Behave As They Do unit v 



Nucleus 



Dotted to indicate 

 great length 



End brush 



Fk;. 258 A neJiron is a coyi/plex cell. MImiI are 

 its parts''' The axofi is covered by a sheath 

 zvljich is not part of the iietiron. 



of vertebrates. Note that the cerebrum 

 in some of the vertebrates is larije in com- 

 parison with the rest of the brain; in 

 other vertebrates it is comparatively 

 small. Since you know something of the 

 activities of the cerebrum, which ani- 

 mals would you predict to be most intel- 

 ligent, judging- from the appearance of 

 the brain? Does your prediction fit what 

 you know of the animal's behavior? 



Fig. 259 Chimpanzee eating. What ivotiJd yon 

 judge about the size and appearance of its cere- 



brUVl? (new YORK ZOOLOGICAL SOCIETY) 



Comparison of the brain of the sheep 

 (Fig. 255) with the diagram of the hu- 

 man brain again shows a difference in 

 size of cerebrum. In fact, the more 

 nearly the cerebral cortex of an animal 

 approaches that of man in size and com- 

 plexity, the more nearly does the ani- 

 mal's behavior resemble man's. You can 

 test your understanding by doing Exer- 

 cise 4. 



Cells of the nervous system. Micro- 

 scopic study of the gray matter of the 

 brain and cord shows that it is made up 

 largely of nerve cells, or neurons (new'- 

 rons). The neuron is the unit of struc- 

 ture of the nervous sy^stem. There are 

 several different kinds of neurons. We 

 shall now study one kind. See Figure 

 258. This neuron consists of a cell body 

 draw n out into two kinds of projections. 

 One of these is the axon, the other the 

 dendrite. Each cell is likely to have two 

 or more dendrites and the dendrites are 

 usually thickly branched, close to the cell 

 body. The name "tlcndrite" comes from 

 the Greek word meanincr tree. The axon 

 is sinele and is unbranched or only 

 sliohtly branched. However, at its ex- 



