626 THE NERVOUS SYSTEM 



The white rami are absent in all the spinal nerves in the regions above 

 the second (occasionally the first) thoracic nerve root, and below the second 

 lumbar nerve root, with the occasional exception of the roots of the third and 

 fourth lumbar nerves. This is a rather restricted field of origin for the pre- 

 ganglionic fibers which compose the white rami. The fibers enter the 



FIG. 418. Scheme of the Constitution and Connections of Gangliated Cord of the 

 Sympathetic. The gangliated cord is indicated on the right, with the arrangement of the 

 fibers arising from ganglion cells. On the left, the roots and trunks of the spinal nerves are 

 shown, with the arrangement of the white ramus communicans above and the gray ramus 

 below. The cells of origin in the ventral cord of the fibers constituting the white ramus are 

 not shown. (Cunningham.) 



adjacent ganglia of the chain to end there or to pass to higher or lower levels 

 or to more peripheral ganglia. 



A peculiarity in the structure of these white medullated visceral nerves is 

 the fineness of their fibers. They are a third or a fourth of the diameter of 

 ordinary medullated fibers, measuring i.S/j. to 2.7^ instead of 14/4 to iq/j.. 



