STRUCTURE OF THE NERVOUS SYSTEM 295 



that pass into and become a part of the nerves of the body ; and third, 

 that connect the opposite sides of these parts together. 



The Peripheral Division. The peripheral division of 

 the nervous system includes all the nervous structures 

 found outside of the brain and spinal cord. These con- 

 sist of the cranial, spinal, and sympathetic nerves, and of 

 various small ganglia, all of which are closely connected 

 with the central system. 



Spinal Nerves and Dorsal- root Ganglia. ^The spinal 

 nerves comprise a group of thirty-one pairs, which connect 

 the spinal cord with different parts of the trunk, with the 

 upper, and with the lower extremities. Each nerve joins 

 the cord by two roots, these being named from their posi- 

 tions the ventral, or anterior, root and the dorsal, or pos- 

 terior, root. The two roots blend together within the 

 spinal cavity to form a single nerve trunk, which passes 

 out between the vertebrae. On the dorsal root of each 

 spinal nerve is a small ganglion which is named, from its 

 position, the dorsal-root ganglion. (Consult Figs. 133 and 

 135, and also Fig. 125.) 



Double Nature of Spinal Nerves. Charles Bell, in 1811, 

 made the remarkable discovery that each spinal nerve is 

 double in function. He found the portion connecting 

 with the cord by the dorsal root to be concerned in the 

 production of feeling and the portion connecting by the 

 ventral root to be concerned in the production of motion. 

 In keeping with these functions, the two divisions of 

 the nerve are made up of different kinds of fibers, as 

 follows : 



1. The dorsal-root divisions, of the fibers of di-axonic 

 neurons, the cell-bodies of which form the dorsal-root 

 ganglia (Fig. 135). 



2. The ventral-root divisions, of the fibers of mon-axonic 



