THE VERTEBRATE BODY 



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in the center of the spinal cord. These spaces are filled with the cere- 

 brospinal fluid which is under pressure and helps hold the soft brain and 

 spinal cord in shape. The fluid also fills the space between the brain 

 and the skull. This helps protect the brain from injury due to blows 

 on the head. It also is found between the spinal cord and the bony 

 vertebral column. 



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Photos by Winchester 



Fig. 22.14. The flexion reflex in the frog. Even though the hind legs and a portion of 



the spinal column are the only parts remaining, this frog still has the power of reflex 



action. When the leg is stimulated with dilute acid, it is drawn up vigorously in 



response. The action was stopped by speed flash. 



There are ten pairs of cranial nerves which are listed in our survey 

 of the parts of the brain. All of these except the tenth run to parts of 

 the head as we have seen. The spinal nerves come out of the spinal 

 cord between the vertebrae. Nerves carry impulses only one way be- 

 cause these impulses can cross the nerve connections in only one direc- 

 tion. Hence, there must be some nerve fibers which bring impulses to 

 the spinal cord or brain and others which carry them away. Those 

 going to the spinal cord or brain are called sensory nerves, whereas 



