THE NERVOUS SYSTEM 



301 



are arranged one on top of the other, these openings form a 

 long tube, the spinal canal. In this is the spinal cord, con- 

 tinuous with the brain above and terminating by dividing into 

 branches in the lumbar vertebra?, Fig. 121. It is nearly uni- 

 form in size throughout its length, though it enlarges somewhat 

 as it merges into the brain and is somewhat larger than else- 

 where in the region between the shoulders and in the lumbar 

 region, or " small of the back." Its average diameter is about 

 three-quarters of an inch. 



Like the brain the cord is protected by two sheaths, the 

 dura mater and the pia mater, which are continuous with 



-Posterior toot 



-Piamafei 



JVerre 

 fibres 



Oral/ 

 Matter 

 Anterior Root 



FIG. 154. A CROSS SECTION OF THE SPINAL CORD 



The white matter is really filled with nerve fibres but in the figure these are shown 



at only one point. 



those of the brain, and like them in every way, save 

 that the dura mater is not grown to the vertebra? as it is to 

 the inner side of the cranial bones. Arachnoid fluid is present 

 and forms a cushion about the cord as it does about the brain. 

 Structure of the Cord. The cord is cylindrical in shape 

 and divided into right and left halves by deep grooves, one 



