THE NERVOUS SYSTEM 293 



all of the motor and sensory fibers of a spinal nerve sepa- 



rate from each other. The motor fibers enter by the an- 



terior root, and the sensory nerves by the posterior root. 



The sensory fibers do not really 



enter the cord but end in a ganglion 



on the posterior root, and the gan- 



glion sends dendrons into the cord 



by the posterior root. The spinal FIG. 199. Cross-section of 



cord extends from the foramen mag- %^ 



" Or great Opening," in the OCci- and Anterior and Posterior 



..-1 i , , Nerve Roots. Ganglion on 



pital bone down to about the second Posterior Root. 

 lumbar vertebra. What parts of a 



vertebra form the canal for the cord ? Is the canal made 

 of bone throughout its length ? How are the vertebrae 

 united with one another? 



539. The spinal cord is about the size of the little finger. 

 It is only about two thirds as large as its tube, so that it is not 

 likely to be injured by bending the backbone. The rest of 

 the space in the canal is taken up by a lymphlike liquid and 

 three membranes called meninges, which form a triple cov- 

 ering for the cord. These membranes extend into the skull 

 and cover the brain. An inflammation of them constitutes 

 a very serious disease called cerebro-spinal meningitis. 



540. A Cross-section of the spinal cord shows that it 

 is a double organ (Fig. 199), the halves being united by a 

 narrow portion ; it shows also that the central part of the 

 cord is of gray matter in the outline of a butterfly, and sur- 

 rounded by a thick layer of white fibers. The gray por- 

 tion is made up of nerve cells that give off fibers, many of 

 which go to the spinal nerves and some go upward in the 

 outer white portion of the cord. The white tract also con- 

 tains motor threads, bringing impulses from the brain to 

 the cells in the gray matter. 



541. Voluntary Motion and Action of the Spinal Cells. 

 The brain sends impulses to the cells in the spinal cord, 



