124 A GUIDE FOR THE STUDY OF ANIMALS 



B. Spinal cord, extending along the spine, giving rise 

 to nerves. 



C. Spinal nerves, ten pairs of nerves which are connected 

 with the cord through dorsal (sensory) and ventral (motor) 

 rodts, and which penetrate the body and its appendages. 

 The first enters the neck; the second and third join and 

 enter the arm ; the fourth to sixth penetrate the skin and 

 muscles of the trunk ; the seventh to ninth join-by a plexus 

 to form the sciatic nerve which supplies the leg, and the 

 tenth enters the posterior portion of the body. 



Questions. 



1. Which lobes of the brain are paired.' Give at least 

 two possible causes or reasons for their double structure. 



2. The optic lobes are connected with the eyes. Com- 

 pare their size with that of the other parts. Of how much 

 use do you think they are to the frog .'' 



3. If folds or convolutions in the surface of the cerebrum 

 indicate intelligence, thoughtfulness, or mind, what do you 

 infer as to the frog's mental condition and power to think .' 



4. Has the frog brain or " brains " .-' Explain. 



5. Name several things done by an animal's brain. 



6. Which of the spinal nerves are specially large .' Why 

 should they be large ? 



7. What advantages are there in the frog's having a dorsal 

 nerve cord instead of a ventral one, as the earthworm has .' 



What would be the effect of cutting or breaking the dorsal root of a 

 spinal nerve? the ventral root? the entire spinal cord? How do you 

 explain the convulsions of the brainless (beheaded) chicken or frog? 



8. What kinds of impulses originate outside of the nerve center, and 

 what kinds in the center? 



9. Sensations reach the cord and brain through the dorsal root of the 

 spinal nerve. What kinds of messages travel through the ventral root, 

 and in what direction do they go ? 



