282 THE NERVOUS SYSTEM 



their union the posterior root passes through a little knot 

 of gray matter, called the spinal ganglion. 



The cells of the anterior horn of the gray matter of the 

 cord are large and branching, and each cell sends off an 

 axis cylinder process which passes out in an anterior nerve 

 root. Careful experiments have shown that the fibers in 

 the posterior nerve roots of spinal nerves arise in the spi- 

 nal ganglion. The nerve fibers which form this posterior 

 root are axis cylinder processes from nerve cells in the 

 spinal ganglion. As they pass out from the ganglion each 

 divides into two branches, one of which goes to form a 

 sensory nerve fiber in the spinal nerve. The other turns 

 back and enters the spinal cord, where it again divides, 

 its fibers ending variously. The main division of the 

 branch from the ganglion passes up to the brain, giving 

 off fine collaterals which end, by " arborizing " (as the 

 brushlike ending is called), round nerve cells at different 

 levels in the cord. In general, the posterior root fibers 

 travel upward mainly in the white columns of the cord, 

 while only a few fibers enter the gray matter of the cord. 



417. Thirty-one pairs of spinal nerves issue from the 

 spinal cord (Fig. 18, p. 28), each nerve containing 

 both afferent, or sensory, nerve fibers from the posterior 

 root, and efferent, or motor, fibers from the anterior root. 

 These fibers remain distinct from each other for their 

 whole length. The spinal nerves are distributed by many 

 branches to the skin and skeletal muscles; they also form 

 connection, by what are called communicating branches, 

 with the ganglia of the sympathetic system (Figs. 128 and 

 136), and are afterward distributed to the viscera. The 

 whole of the sympathetic system may be regarded as 

 simply the development of these communicating branches 

 from certain spinal nerves. 



