Cerebrum 



PROBLEM 2. Why Co?nplex Behavior Is Possible 



Protection of the nerve centers. The 



nerve centers of the central nervous sys- 

 tem are well protected in man by bony 

 coverings and by membranes. Covering 

 the brain and spinal cord are membranes 

 called the meninges (men-in'jees). A 

 liquid between the membranes and in 

 brain cavities acts as a shock absorber. 

 Outside the membranes of the brain is a 

 cranium or skull made of bones tightly 

 joined. Outside the cranium is the scalp. 

 If your school has a human skeleton ex- 

 amine the skull. If not, you can learn a 

 great deal about your own skull by ex- 

 amining the skull of a cat or dog. 



The spinal cord is almost as well pro- 

 tected as the brain. Besides the mem- 

 branes and the liquid, the cord is pro- 

 tected by the spinal column or backbone. 

 The backbone is not one bone but a 

 series of small bones, the vertebrae (ver'- 

 teh-bree), which are held together by 

 ligaments. The column is sometimes 

 called the vertebral (ver'teh-bral) col- 

 umn. Because the bones composing it are 

 not rigidly joined to each other, the 

 backbone is flexible. The spinal cord ends 

 about three quarters of the way down 

 the spinal column. All of this will be 

 clearer to you if you do Exercises i 

 and 2. 



The brain and spinal cord. The human 

 brain consists of several regions, three of 

 which are easily distinguished: the cere- 

 brum (ser'e-brum), the cerebellum 

 (ser-e-bel'lum), and the medulla (med- 

 duU'a). The cerebrum is by far the larg- 

 est part of the human brain; it lies di- 

 rectly under the top of the skull. The 

 cerebellum is tucked in under the back 

 of the cerebrum next to the medulla. 

 The medulla or brain stem looks like a 



Fig. 255 A sheep's brain. The front end (left) 

 is the cerebnn/i. }Vhat are the other two parts? 

 The spinal cord is attached at the right, (ward's 



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Pineal gland 



Cortex 



Cerebrum 



Cerebellum 



Pituitary gland 

 Medulla 



Spinal cord 



White matter 



Fig. 256 A section through the middle pj the 

 human brain. Coinpare it with the brain of the 

 sheep. Which of the three regions of the brain 

 have convolutions? 



widened out spinal cord; in fact, one 

 might consider the cord as an extension 

 of the brain. This shows clearly in Fig- 

 ure 256. You will find it helpful to study 

 a sheep's brain, which is somewhat like 

 our brain in structure (Exercise 3). 



The cerebrum has a right and a left 

 half. There is a deep cut (fissure) be- 

 tween the two halves that almost com- 

 pletely separates them. If you cut all the 

 way down through the brain between 

 these halves you will discover in the very 



