Nervous System. 41 



Afferent and Efferent Nerve Fibers. Nerve fibers that 

 carry impulses toward tr^e spinal cord or brain are called 

 afferent nerve fibers. Fibers that convey impulses from 

 the brain or spinal cord are efferent nerve fibers. 



Cross-section of the Spinal Cord. If a thin slice of 

 the spinal cord be made as shown in Fig. 25, it will be 

 seen that the central part is darker in color than the outer 

 part. The central part is known as the gray matter, in 

 distinction from the rest, which is called the white matter. 



Dorsal Septum 



Dorsal or Sensor 

 Root 



Ventral Fissure 

 Ventral or Motor Root 



Fig. 25. Cross-section of Spinal Cord. 



The white matter of the nervous system is made up of 

 nerve fibers, whose structure and use we have just con- 

 sidered. But the gray matter has a different structure and 

 a different function. Instead of being made up mainly of 

 fibers, it is composed of cells, one of the forms of which is 

 represented in Fig. 26. Some of the branches of these 

 cells are continued, and become the axis cylinders of 

 nerves, and it is believed that every nerve fiber begins as 

 a branch of some nerve cell. 



